[llvm-cov] Support directory layout in coverage reports

[AtomicGu]

Summary

This is a GSoC 2023 project (discourse link).

llvm-cov currently generates a single top-level index HTML file, which causes rendering scalability issues in large projects. This patch adds support for hierarchical directory structure into the HTML reports to solve scalability issues by introducing the following changes:

• Added a new command line option --show-directory-coverage for llvm-cov show. It works both for --format=html and --format=text.
• Two new classes: CoveragePrinterHTMLDirectory and CoveragePrinterTextDirectory was added to support the new option.
• A tool class DirectoryCoverageReport was added to support the two classes above.
• Updated the document.
• Added a new regression test for --show-directory-coverage.

Here is a coverage report preview of LLVM:

report1.7z109 MBDownload

[gulfem]

It can be useful to add an option for hierarchical directory structure like other coverage viewing options (https://github.com/llvm/llvm-project/blob/main/llvm/tools/llvm-cov/CoverageViewOptions.h#L30) to be able to turn it on/off:

llvm/tools/llvm-cov/CoverageReport.h

47 ↗

(On Diff #524988)

Can you please make sure that you only format the lines that you change, and not format the file whole file? This makes it easier for the reviewers to see the actual changes instead of formatting changes? AFAICT, most of the changes in CoverageReport.h and CoverageReport.cpp are formatting changes.

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

498

When the directory contains a single file, it is added to Files, but all the FileReports for Files are previously generated at line 494. So, there won't be any report generated for a single file in a directory.

[AtomicGu]

llvm/tools/llvm-cov/CoverageReport.h

47 ↗

(On Diff #524988)

Can you please make sure that you only format the lines that you change, and not format the file whole file? This makes it easier for the reviewers to see the actual changes instead of formatting changes? AFAICT, most of the changes in CoverageReport.h and CoverageReport.cpp are formatting changes.

47 ↗

(On Diff #524988)

Sorry, this is done by clang-format automatically. I will figure this out in the next commit.

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

498

Thank you! You are right, this is a mistake. I was too careless. I will solve this in the next commit.

[gulfem]

llvm/tools/llvm-cov/SourceCoverageViewHTML.h

56

createTopLevelIndexFile might be a better name.

[AtomicGu]

• [llvm-cov] Finish my new CoveragePrinter.
• [llvm-cov] Manual test passed

[AtomicGu]

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

498

The problem has been fixed in the new implementation.

llvm/tools/llvm-cov/SourceCoverageViewHTML.h

56

This function is to create index file in each subdirectory recursivly. I'm not sure the name "TopLevel" is more proper?

[AtomicGu]

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

543

This line has changed to LCP = sys::path::parent_path(SourceFiles.front()).size() + 1;

[AtomicGu]

• [llvm-cov] Add navigation link to the title

[gulfem]

llvm/tools/llvm-cov/SourceCoverageView.cpp

83

Some llvm-cov tests like llvm-cov::branch-c-general.test are failing in the presubmit tests. The reason is that you are changing the default behavior and some tests rely on that behavior. As a transitioning step, I would suggest adding a new option like showDirectoryCoverage into https://github.com/llvm/llvm-project/blob/main/llvm/tools/llvm-cov/CoverageViewOptions.h#L30, and using the associated class based on that option like:

if (Opts.ShowDirectoryCoverage)
  return std::make_unique<CoveragePrinterHTMLDirectory>(Opts);
else
  return std::make_unique<CoveragePrinterHTML>(Opts);

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

456–457

Please remove the whitespace at the end of this line.

624

I would suggest using the words CurrentFiles, CurrentDirs and CurrentTotals.

659

Make sure that else starts after this curly base as stated in LLVM Coding Standards.
https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements

722

Use a curly brace for the else part make it uniform.

730

Can you use a long name here? What does buildNavLink mean? Try to use a full name unless the abbreviation is clear.

885

This is another formatting change.

[AtomicGu]

• [llvm-cov] Finish the new CoveragePrinterHTMLDirectory.
• [llvm-cov] The new CoveragePrinterHTMLDirectory passed manual test.
• [llvm-cov] Add navigation links to the title of the index file.
• [llvm-cov] Add additional command line options.
• [llvm-cov] Accept advices from Gulfem's review.

[AtomicGu]

Here is a preview coverage report for llvm-cov llvm-profdata lli and llc :

report.tar.gz153 MBDownload

[yurybura]

In D151283#4420869, @AtomicGu wrote:

Here is a preview coverage report for llvm-cov llvm-profdata lli and llc :

report.tar.gz153 MBDownload

Provided preview contains wrong relative path to style.css. For example, it should be href='../../../../../style.css' in file coverage/root/GSoC/llvm-project/llvm/index.html.

[AtomicGu]

• [llvm-cov] Fix links to CSS in index files.
• [llvm-cov] Improve the title of index files.

[AtomicGu]

Reverse formatting changes.

[AtomicGu]

In D151283#4421727, @yurybura wrote:

In D151283#4420869, @AtomicGu wrote:

Here is a preview coverage report for llvm-cov llvm-profdata lli and llc :

report.tar.gz153 MBDownload

Provided preview contains wrong relative path to style.css. For example, it should be href='../../../../../style.css' in file coverage/root/GSoC/llvm-project/llvm/index.html.

Thank you. This issue has been fixed. I was testing by python -m http.server remotely and not aware of this.

[AtomicGu]

• [llvm-cov] Update documents.

[AtomicGu]

• [llvm-cov] Fix a format issue of git-clang-format.

[AtomicGu]

@davidxl @phosek @gulfem
I removed the WIP tag. Please review this! 😀
The new tests will be added in the comming few days.

[phosek]

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

530–536

This code is duplicated between CoveragePrinterHTML::createIndexFile and CoveragePrinterHTMLDirectory::createIndexFile, could it be extracted into a helper function?

592–617

This code is duplicated between CoveragePrinterHTML::createIndexFile and CoveragePrinterHTMLDirectory::createSubIndexFile, could it be extracted into one or more helper functions?

685–706

This code is duplicated between CoveragePrinterHTML::createIndexFile and CoveragePrinterHTMLDirectory::createSubIndexFile, could it be extracted into one or more helper functions?

[AtomicGu]

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

530–536

This can be extracted to a helper method of CoveragePrinterHTML.

Personally, I tolerate code repetition for modification flexibility if it's not more than twrice. If it is not welcomed in LLVM community, I will fix it.

[AtomicGu]

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

685–706

These codes are tightly coupled with variables around them. It may needs more boilerplate code than the code extracted. I don't think it will make the code easier to read to extract them into a common helper function.

[AtomicGu]

This comment was removed by AtomicGu.

[AtomicGu]

• [llvm-cov] Extract some common code to 2 methods.

[AtomicGu]

• [llvm-cov] Add new tests for directory coverage.

[phosek]

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

430

This is a just a suggestion, but I think that emitPrelude might be a better name.

[AtomicGu]

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

430

This name has been used by another function in the anonymous namespace in SourceCoverageViewHTML.cpp. So I chose a different name to avoid confusion, although no conflict would be caused if I used emitPrelude since the arguments are different.

[phosek]

llvm/test/tools/llvm-cov/directory_coverage.test

2

Your test cannot assume that clang is available on the host since Clang is not one of LLVM's prerequisites, and we want LLVM to remain compiler agnostic. That's why you're seeing the issue on Windows pre-merge builders: Clang happens to be available on these builders, but it doesn't include the profile runtime, which is perfectly valid because we don't require it.

What we do instead is to either include raw profile in the .proftext textual format and then convert it to .profdata using llvm-profdata or include .profdata directly, plus include coverage mapping information in the textual format as .covmapping, and then run llvm-covas part of the test. We also include instructions for recreating the .profdata and .covmapping files inside the test.

Here are some examples as an inspiration:

https://github.com/llvm/llvm-project/blob/96ae0851c26237378fa1280b0a9ad713e1b72bdb/llvm/test/tools/llvm-cov/coverage-prefix-map.test
https://github.com/llvm/llvm-project/blob/96ae0851c26237378fa1280b0a9ad713e1b72bdb/llvm/test/tools/llvm-cov/multiple-objects.test

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

430

I see, how about emitReportHeader?

605–606

LLVM's StringMap is more efficient than std::map<StringRef, ...>.

606

We prefer LLVM SmallVector unless there's a specific reason for using std::vector.

695–706

Can you add some comments to explain the logic in this block?

708

Use llvm::SmallString.

712–713

Use llvm::sys::path::append appending to a path.

735

Use llvm::SmallString here as well.

[gulfem]

Can we simplify the commit message? We do not need all the details in the commit message as we have them in the Discourse discussion. I would suggest something like: "llvm-cov currently generates a single top-level index HTML file, which causes rendering scalability issues in large projects. This patch adds support for hierarchical directory structure into the HTML reports to solve scalability issues."

llvm/docs/CommandGuide/llvm-cov.rst

256

How about we stop the description before the parenthesis? I think "impossible to read" is a strong statement.

[gulfem]

llvm/test/tools/llvm-cov/directory_coverage.test

5

I would use b0 for this directory name to be consistent overall the test.

[AtomicGu]

llvm/test/tools/llvm-cov/directory_coverage.test

2

I tried this today. llvm-cov convert-for-testing works well for single source file program. But when I used it for multiple source files, the generated .covmapping file is failed to be load by llvm-cov show:

error: Failed to load coverage: 'main.covmapping': Malformed coverage data

The commands used is same with those in coverage-prefix-map.test:

cp %S/Inputs/directory_coverage -r . /tmp
cd /tmp/directory_coverage
clang -fprofile-instr-generate -mllvm -enable-name-compression=false \
  -fcoverage-mapping -fcoverage-prefix-map=$PWD=. -o main \
  main.cc a0/a1/a2.cc b0/b1_1.cc b0/b1_2.cc c0/c1/c2_1.cc c0/c1/c2_2.cc
LLVM_PROFILE_FILE="main.raw" ./main
llvm-profdata merge main.raw -o main.profdata
llvm-cov convert-for-testing ./main -o main.covmapping
rm main main.raw

llvm-cov report -path-equivalence=.,/tmp/coverage_prefix_map \
  -instr-profile main.profdata main.covmapping

Is this a bug or a designed behavior? Can I just include the program file directly for now?

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

708

I believe that S is not small. It contains hyperlinks <a href="..."> </a> for each level. So I think it can easily exceed 64 chars or more in large projects.

Also it's the return value. I choose std::string for the return type because the original code uses that. For example, CoveragePrinterHTML::buildLinkToFile returns a std::string.

712–713

S is not a path. It's a html snippet like:

<a href="../../index.html">/root</a>/<a href="./index.html">c0/c1</a>/

Here I just need native separators to show between the <a> tags.

735

I use std::string here because the parameters of the function a(..., ...) is const std::string &. In fact, the two helper function tag and a all uses const std::string &. Do I need to refactor them?

[AtomicGu]

In D151283#4503401, @gulfem wrote:

Can we simplify the commit message? We do not need all the details in the commit message as we have them in the Discourse discussion. I would suggest something like: "llvm-cov currently generates a single top-level index HTML file, which causes rendering scalability issues in large projects. This patch adds support for hierarchical directory structure into the HTML reports to solve scalability issues."

Do you mean I should squash the commits into one single commit? I thought Phabricator would do this for me as I always use arc diff with origin/main. I'm not sure what should I do.

[AtomicGu]

• [llvm-cov] Take advices from phosek and gulfem.

[AtomicGu]

• [llvm-cov] --show-directory_coverage for text.

I refactored the HTML printer and extract the recursion part into a common template method pattern class named "DirectoryCoverageReport".

Manual test is passed (both on linux and windows).

[gulfem]

In D151283#4505153, @AtomicGu wrote:

In D151283#4503401, @gulfem wrote:

Can we simplify the commit message? We do not need all the details in the commit message as we have them in the Discourse discussion. I would suggest something like: "llvm-cov currently generates a single top-level index HTML file, which causes rendering scalability issues in large projects. This patch adds support for hierarchical directory structure into the HTML reports to solve scalability issues."

Do you mean I should squash the commits into one single commit? I thought Phabricator would do this for me as I always use arc diff with origin/main. I'm not sure what should I do.

Current commit message looks like this:

[llvm-cov] Support a Hierarchical Structure for HTML Coverage Report Generating
   
> This is a GSoC23 project ([discourse link](https://discourse.llvm.org/t/coverage-support-a-hierarchical-directory-structure-in-generated-coverage-html-reports/68239/7)).

Clang supports [source-based coverage](https://clang.llvm.org/docs/SourceBasedCodeCoverage.html) that shows which lines of code are covered by the executed tests. It uses [llvm-profdata](https://llvm.org/docs/CommandGuide/llvm-profdata.html) and [llvm-cov](https://llvm.org/docs/CommandGuide/llvm-cov.html) tools to generate coverage reports. llvm-cov currently generates a single top-level index HTML file. For example, a single top-level directory [code coverage report](https://lab.llvm.org/coverage/coverage-reports/index.html) for LLVM repo is published on a coverage bot. Top-level indexing causes rendering scalability issues in large projects, such as [Fuchsia](https://fuchsia.dev). The goal of this project is to generate a hierarchical directory structure in generated coverage html reports to match the directory structure and solve scalability issues. Chromium uses its own [post-processing tools](https://source.chromium.org/chromium/chromium/tools/build/+/main:recipes/recipe_modules/code_coverage/resources/generate_coverage_metadata.py) to show a per-directory hierarchical structure for [coverage results](https://analysis.chromium.org/coverage/p/chromium). Similarly, [Lcov](https://llvm.org/reports/coverage/index.html), which is a graphical front-end [Gcov](https://gcc.gnu.org/onlinedocs/gcc/Gcov.html), provides a one-level directory structure to display coverage results.

Changes to the code:

- Added a new CoveragePrinterHTMLDirectory by inheriting the original CoveragePrinterHTML.
- Added a new command line option `--show-directory-coverage` to select the new CoveragePrinterHTMLDirectory.
- Updated the coverage report generating script of LLVM itself to use the new option.
- Updated the document.
- Added a new regression test for `--show-directory-coverage`.

Currently the new option only works with `--format=html`. There is a plan to support `--format=text` too in the future.

I'm suggesting removing the first paragraph:

[llvm-cov] Support directory layout in coverage reports 
    
This is a GSoC 2023 project ([discourse link](https://discourse.llvm.org/t/coverage-support-a-hierarchical-directory-structure-in-generated-coverage-html-reports/68239)).
 
llvm-cov currently generates a single top-level index HTML file, which causes rendering scalability issues in large projects. This patch adds support for hierarchical directory structure into the HTML reports to solve scalability issues by introducing the following changes:
 - Added a new CoveragePrinterHTMLDirectory by inheriting the original CoveragePrinterHTML.
 - Added a new command line option `--show-directory-coverage` to select the new CoveragePrinterHTMLDirectory.
 - Updated the coverage report generating script of LLVM itself to use the new option.
 - Updated the document.
 - Added a new regression test for `--show-directory-coverage`.
 
 Currently the new option only works with `--format=html`. There is a plan to support `--format=text` too in the future.

[AtomicGu]

• [llvm-cov] Add regression tests.

llvm-cov convert-for-testing not works for me. When I use it for program with multiple source files, the generated .covmapping file can not be read back by llvm-cov:

error: Failed to load coverage: 'main.covmapping': Malformed coverage data

So, I just include two program files (one for Linux and the other one for Windows) directly.

[AtomicGu]

@gulfem OK, I have updated the title and summary. I hope this will do.

[AtomicGu]

• [llvm-cov] Add regression tests.

I think they can pass the pre-merge checks now.

[phosek]

llvm/test/tools/llvm-cov/directory_coverage.test

2

That's not expected, I'd try to investigate this further. Did you build all the tools yourself from the latest revision? Did you enable zlib?

We definitely don't want to include the program file directly since that's going to be in the Git history forever and will take extra space.

[AtomicGu]

llvm/test/tools/llvm-cov/directory_coverage.test

2

I pulled the newest commit and tried again. The problem still exists.

I checked the CMakeCache.txt. LLVM_ENABLE_ZLIB is ON.

Here are the source code, program, profdata and covmapping files:

directory_coverage.tar.gz21 KBDownload

Commands to reproduce the issue:

llvm-cov report main.covmapping --instr-profile main.profdata

[gulfem]

llvm/test/tools/llvm-cov/directory_coverage.test

2

I was able to reproduce it locally. AFAICT, the error is thrown in the following line:
https://github.com/llvm/llvm-project/blob/main/llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp#L720

The issue seems about the filenames. I did not have much time to dig into that yet, but we might be uncovering an issue about convert-for-testing flag in llvm-cov.

[gulfem]

In D151283#4511131, @AtomicGu wrote:

@gulfem OK, I have updated the title and summary. I hope this will do.

Thanks, that looks better.

[AtomicGu]

• [llvm-cov] Add regression tests.

[AtomicGu]

• [llvm-cov] Use .covmapping files for tests.

[AtomicGu]

@gulfem @phosek Hi. Do I need to update the Release Notes to indicate the new llvm-cov functionality?

[AtomicGu]

This comment was removed by AtomicGu.

[AtomicGu]

Update differential based on D156611.

[AtomicGu]

llvm/tools/llvm-cov/CoverageReport.cpp

578 ↗

(On Diff #550604)

@gulfem @phosek Any advises for this problem?

[AtomicGu]

Rebase to D156611.

[AtomicGu]

Linux tests passed.

[AtomicGu]

• Pass windows tests and fix COFF support of convert-for-testing.
• Disable name compression of directory_coverage.linux.test

[AtomicGu]

@phosek @gulfem All tests have been passed. The patch is ready to land, unless you may want to check this problem: https://reviews.llvm.org/D151283#inline-1531509 .

[gulfem]

llvm/tools/llvm-cov/CoverageReport.cpp

578 ↗

(On Diff #550604)

Can you use sys::path::native to convert the path to native path? I looked at native_separators.c test:
https://github.com/llvm/llvm-project/blob/main/llvm/test/tools/llvm-cov/native_separators.c

It's a Windows test, but its *.covmapping file input does not include backslashes for the filenames because it converts the path read to native path:
https://github.com/llvm/llvm-project/blob/main/llvm/test/tools/llvm-cov/Inputs/native_separators.covmapping

So, what I was thinking is that you can have two separate tests directory_coverage.linux.test and directory_coverage.win.test, but they use the same inputs for *.profdata and *.covmapping files. Do you think that would work? If not, having two separate files with their own inputs should be fine.

[AtomicGu]

llvm/tools/llvm-cov/CoverageReport.cpp

578 ↗

(On Diff #550604)

Actually there's another difference between the two tests: the linux test uses absolute path in the .covmapping file but the windows test uses relative path.

The key problem here is: do we want to support generating reports for inputs of different platforms? I think native_separators.c answers "yes", so I will follow it.

[phosek]

llvm/tools/llvm-cov/CoverageReport.cpp

578 ↗

(On Diff #550604)

Yes, I think so, we don't currently restrict handling of inputs to a particular platform so this would be a regression. I agree that using sys::path::native to convert all paths to native format is likely the right approach.

[AtomicGu]

• Use same input for linux and windows test.

[AtomicGu]

• Use sys::path::native to suport showing on different platforms.

[phosek]

llvm/docs/CommandGuide/llvm-cov.rst

254–256

I think this would be a better summary of what this flag does.

llvm/tools/llvm-cov/CoverageReport.cpp

559 ↗

(On Diff #552263)

Rather than using an assert which would be compiled out when assertions are disabled, I'd consider returning an Error in this case.

llvm/tools/llvm-cov/CoverageReport.h

105 ↗

(On Diff #552263)

130 ↗

(On Diff #552263)

Is there a reason for using an operator here rather than a regular method?

[AtomicGu]

llvm/tools/llvm-cov/CoverageReport.cpp

559 ↗

(On Diff #552263)

This assertion is to prevent coding errors. In a correctly written program, it cannot fail. All its callers, prepareSubDirectoryReports and prepareDirectoryReports, won't pass an empty Files to it. The only exception is that the prepareDirectoryReports is used incorrectly. So, I think returning an` Error` is unnecessary and might cause runtime overhead, potentially delaying the discovery of bugs.

llvm/tools/llvm-cov/CoverageReport.h

130 ↗

(On Diff #552263)

No. I was thinking this class would be used like functions, but so far it's not. I'm going to rename it to generateSubDirectoryReport.

[AtomicGu]

• Rename operator()() and revise comments.

[AtomicGu]

@phosek @gulfem The tests finished. I think we can land the patch now. One test failed because of timeout, but it has nothing to do with this patch.

[AtomicGu]

Retry per-merge checks.

[AtomicGu]

Acturally, I have got the commit access from Chris (😊). But I'm not sure how to do that. Accroding to the documentation: https://llvm.org/docs/Contributing.html#commit-from-git . Should I use git push to commit? How to squash all the commits into a single one and set the commit message correctly? Does arc have some automatic commands?

[phosek]

llvm/utils/prepare-code-coverage-artifact.py

70

Would it be possible to drop this change from this patch and do it as a separate change? That way, if the coverage bot breaks for some reason, we can only revert a much smaller change rather than having to revert this entire change.

[AtomicGu]

llvm/utils/prepare-code-coverage-artifact.py

70

Nice advice. So thoughtful of you! I will do this.

[AtomicGu]

• Revert llvm/utils/prepare-code-coverage-artifact.py

llvm/docs/CommandGuide/llvm-cov.rst

  the character where the region starts. Defaults to false.
 
 .. option:: -show-line-counts-or-regions
 
  Show the execution counts for each line if there is only one region on the
  line, but show the individual regions if there are multiple on the line.
  Defaults to false.
 
+.. option:: -show-directory-coverage
+
+ Generate index files in each level of the directory to show the coverage.
+ Defaults to false and a single index file will be generated in the output
+ directory.
+
 .. option:: -use-color
 
  Enable or disable color output. By default this is autodetected.
 
 .. option:: -arch=[*NAMES*]
 
  Specify a list of architectures such that the Nth entry in the list
  corresponds to the Nth specified binary. If the covered object is a universal

llvm/test/tools/llvm-cov/Inputs/directory_coverage/a0/a1/a2.cc

+#include "../../header.h"
+
+template<>
+bool equal<int>(int a, int b)
+{
+  return a == b;
+}

llvm/test/tools/llvm-cov/Inputs/directory_coverage/b/b1.h

+int add(int a, int b);
+
+int sub(int a, int b);

llvm/test/tools/llvm-cov/Inputs/directory_coverage/b/b1_1.cc

+#include "b1.h"
+
+int add(int a, int b) {
+  return a + b;
+}

llvm/test/tools/llvm-cov/Inputs/directory_coverage/b/b1_2.cc

+#include "b1.h"
+
+int sub(int a, int b) {
+  return a - b;
+}

llvm/test/tools/llvm-cov/Inputs/directory_coverage/c0/c1/c2.h

+int mul(int a, int b);
+
+int div(int a, int b);
+
+#ifdef DEF
+int div(int a, int b) {
+  return a / b;
+}
+#endif

llvm/test/tools/llvm-cov/Inputs/directory_coverage/c0/c1/c2_1.cc

+int mul(int a, int b) {
+  return a * b;
+}

llvm/test/tools/llvm-cov/Inputs/directory_coverage/c0/c1/c2_2.cc

+#define DEF
+#include "c2.h"

llvm/test/tools/llvm-cov/Inputs/directory_coverage/header.h

+#include "b/b1.h"
+#include "c0/c1/c2.h"
+
+template<typename T>
+bool equal(T a, T b);

llvm/test/tools/llvm-cov/Inputs/directory_coverage/main.cc

+#include "header.h"
+
+int main() {
+  int a = 3;
+  int b = 4;
+  int c = c = add(a, b);
+  int d = mul(a, b);
+  return equal(a, sub(c, b)) - equal(a, div(d, b));
+}

llvm/test/tools/llvm-cov/directory_coverage.test

+// RUN: mkdir -p %t && clang -fprofile-instr-generate=%t/default.profraw -fcoverage-mapping -o %t/main.out \
+// RUN:   %S/Inputs/directory_coverage/main.cc \
+// RUN:   %S/Inputs/directory_coverage/a0/a1/a2.cc \
+// RUN:   %S/Inputs/directory_coverage/b/b1_1.cc \
+// RUN:   %S/Inputs/directory_coverage/b/b1_2.cc \
+// RUN:   %S/Inputs/directory_coverage/c0/c1/c2_1.cc \
+// RUN:   %S/Inputs/directory_coverage/c0/c1/c2_2.cc
+// RUN: %t/main.out
+// RUN: llvm-profdata merge %t/default.profraw -o %t/profdata
+// RUN: llvm-cov show %t/main.out --instr-profile %t/profdata --format=html --show-directory-coverage -o %t/report
+// RUN: cat %t/report/index.html | FileCheck %s --check-prefix=ROOT --allow-empty
+// RUN: cat %t/report/coverage/%:S/Inputs/directory_coverage/index.html | FileCheck %s --check-prefix=TOP
+// RUN: cat %t/report/coverage/%:S/Inputs/directory_coverage/b/index.html | FileCheck %s --check-prefix=B
+// RUN: cat %t/report/coverage/%:S/Inputs/directory_coverage/c0/c1/index.html | FileCheck %s --check-prefix=C1
+
+// ROOT: coverage{{.*}}Inputs{{/|\\}}directory_coverage{{/|\\}}index.html
+
+
+// TOP: a0{{/|\\}}a1{{/|\\}}a2.cc
+// TOP: b{{/|\\}}
+// TOP-NOT: b1_1.cc
+// TOP-NOT: b1_2.cc
+// TOP: c0{{/|\\}}c1{{/|\\}}
+// TOP-NOT: c2_1.cc
+// TOP-NOT: b2_2.cc
+// TOP: main.cc
+
+
+// B: href={{"|'}}..{{/|\\}}index.html{{"|'}}
+// B: b1_1.cc
+// B: b1_2.cc
+
+
+// C1: href={{"|'}}..{{/|\\}}..{{/|\\}}index.html{{"|'}}
+// C1: c0{{/|\\}}c1
+// C1: c2.h
+// C1: c2_1.cc

llvm/tools/llvm-cov/CodeCoverage.cpp

int CodeCoverageTool::doShow(int argc, const char **argv,
   cl::opt<bool> ShowExpansions("show-expansions", cl::Optional,
                                cl::desc("Show expanded source regions"),
                                cl::cat(ViewCategory));
 
   cl::opt<bool> ShowInstantiations("show-instantiations", cl::Optional,
                                    cl::desc("Show function instantiations"),
                                    cl::init(true), cl::cat(ViewCategory));
 
+  cl::opt<bool> ShowDirectoryCoverage("show-directory-coverage", cl::Optional,
+                                      cl::desc("Show directory coverage"),
+                                      cl::cat(ViewCategory));
+
   cl::opt<std::string> ShowOutputDirectory(
       "output-dir", cl::init(""),
       cl::desc("Directory in which coverage information is written out"));
   cl::alias ShowOutputDirectoryA("o", cl::desc("Alias for --output-dir"),
                                  cl::aliasopt(ShowOutputDirectory));
 
   cl::opt<uint32_t> TabSize(
       "tab-size", cl::init(2),
ViewOpts.ShowLineStats = ShowLineExecutionCounts.getNumOccurrences() != 0 ||
                            !ShowRegions || ShowBestLineRegionsCounts;
   ViewOpts.ShowRegionMarkers = ShowRegions || ShowBestLineRegionsCounts;
   ViewOpts.ShowExpandedRegions = ShowExpansions;
   ViewOpts.ShowBranchCounts =
       ShowBranches == CoverageViewOptions::BranchOutputType::Count;
   ViewOpts.ShowBranchPercents =
       ShowBranches == CoverageViewOptions::BranchOutputType::Percent;
   ViewOpts.ShowFunctionInstantiations = ShowInstantiations;
+  ViewOpts.ShowDirectoryCoverage = ShowDirectoryCoverage;
   ViewOpts.ShowOutputDirectory = ShowOutputDirectory;
   ViewOpts.TabSize = TabSize;
   ViewOpts.ProjectTitle = ProjectTitle;
 
   if (ViewOpts.hasOutputDirectory()) {
     if (auto E = sys::fs::create_directories(ViewOpts.ShowOutputDirectory)) {
       error("Could not create output directory!", E.message());
       return 1;

llvm/tools/llvm-cov/CoverageViewOptions.h

struct CoverageViewOptions {
   bool ShowBranchCounts;
   bool ShowBranchPercents;
   bool ShowExpandedRegions;
   bool ShowFunctionInstantiations;
   bool ShowFullFilenames;
   bool ShowBranchSummary;
   bool ShowRegionSummary;
   bool ShowInstantiationSummary;
+  bool ShowDirectoryCoverage;
   bool ExportSummaryOnly;
   bool SkipExpansions;
   bool SkipFunctions;
   bool SkipBranches;
   OutputFormat Format;
   BranchOutputType ShowBranches;
   std::string ShowOutputDirectory;
   std::vector<std::string> DemanglerOpts;

llvm/tools/llvm-cov/SourceCoverageView.h

public:
   using OwnedStream = std::unique_ptr<raw_ostream, StreamDestructor>;
 
 protected:
   const CoverageViewOptions &Opts;
 
   CoveragePrinter(const CoverageViewOptions &Opts) : Opts(Opts) {}
 
   /// Return `OutputDir/ToplevelDir/Path.Extension`. If \p InToplevel is
-  /// false, skip the ToplevelDir component. If \p Relative is false, skip the
+  /// true, skip the ToplevelDir component. If \p Relative is true, skip the
   /// OutputDir component.
   std::string getOutputPath(StringRef Path, StringRef Extension,
                             bool InToplevel, bool Relative = true) const;
 
   /// If directory output is enabled, create a file in that directory
   /// at the path given by getOutputPath(). Otherwise, return stdout.
   Expected<OwnedStream> createOutputStream(StringRef Path, StringRef Extension,
                                            bool InToplevel) const;

llvm/tools/llvm-cov/SourceCoverageView.cpp

 }
 
 std::unique_ptr<CoveragePrinter>
 CoveragePrinter::create(const CoverageViewOptions &Opts) {
   switch (Opts.Format) {
   case CoverageViewOptions::OutputFormat::Text:
     return std::make_unique<CoveragePrinterText>(Opts);
   case CoverageViewOptions::OutputFormat::HTML:
+    if (Opts.ShowDirectoryCoverage)
+      return std::make_unique<CoveragePrinterHTMLDirectory>(Opts);
     return std::make_unique<CoveragePrinterHTML>(Opts);
   case CoverageViewOptions::OutputFormat::Lcov:
     // Unreachable because CodeCoverage.cpp should terminate with an error
     // before we get here.
     llvm_unreachable("Lcov format is not supported!");
   }
   llvm_unreachable("Unknown coverage output format!");
 }
 

llvm/tools/llvm-cov/SourceCoverageViewHTML.h

 #define LLVM_COV_SOURCECOVERAGEVIEWHTML_H
 
 #include "SourceCoverageView.h"
 
 namespace llvm {
 
 using namespace coverage;
 
+class ThreadPool;
+
 struct FileCoverageSummary;
 
 /// A coverage printer for html output.
 class CoveragePrinterHTML : public CoveragePrinter {
 public:
   Expected<OwnedStream> createViewFile(StringRef Path,
                                        bool InToplevel) override;
 
   void closeViewFile(OwnedStream OS) override;
 
   Error createIndexFile(ArrayRef<std::string> SourceFiles,
                         const coverage::CoverageMapping &Coverage,
                         const CoverageFiltersMatchAll &Filters) override;
 
   CoveragePrinterHTML(const CoverageViewOptions &Opts)
       : CoveragePrinter(Opts) {}
 
+protected:
+  Error emitStyleSheet();
+  void emitReportHeader(raw_ostream &OSRef, const std::string &Title);
+
 private:
   void emitFileSummary(raw_ostream &OS, StringRef SF,
                        const FileCoverageSummary &FCS,
                        bool IsTotals = false) const;
   std::string buildLinkToFile(StringRef SF,
                               const FileCoverageSummary &FCS) const;
 };
 
+/// A coverage printer for html output, but generates index files in every
+/// subdirectory to show a hierarchical view.
+class CoveragePrinterHTMLDirectory : public CoveragePrinterHTML {
+public:
+  using CoveragePrinterHTML::CoveragePrinterHTML;
+
+  Error createIndexFile(ArrayRef<std::string> SourceFiles,
+                        const coverage::CoverageMapping &Coverage,
+                        const CoverageFiltersMatchAll &Filters) override;
+
+private:
+  // The two member variables are used for avoiding being passed
+  // repeatedly in recursive calling of createSubIndexFile.
+  const coverage::CoverageMapping *CoverageNow;
+  const CoverageFiltersMatchAll *FiltersNow;
+
+  // For calling CoverageReport::prepareSingleFileReport asynchronously
+  // in createSubIndexFile.
+  ThreadPool *PoolNow;
+
+  // Track the report levels from the root to the current directory in
+  // createSubIndexFile. Note that one report level may correspond to multiple
+  // directory levels as we omit directories which have only one subentry.
+  // Each value in the stack corresponds to a LCP prefix length length of LCP.
+  // [0] is the root LCP length.
+  std::vector<unsigned> LCPStack;
+
+  Error createSubIndexFile(const ArrayRef<StringRef> &Files,
+                           FileCoverageSummary *Totals);
+  std::string buildTitleLinks(StringRef LCPath) const;
+  std::string buildRelLinkToFile(StringRef RelPath, bool IsDir) const;
+};
+
 /// A code coverage view which supports html-based rendering.
 class SourceCoverageViewHTML : public SourceCoverageView {
   void renderViewHeader(raw_ostream &OS) override;
 
   void renderViewFooter(raw_ostream &OS) override;
 
   void renderSourceName(raw_ostream &OS, bool WholeFile) override;
 

llvm/tools/llvm-cov/SourceCoverageViewHTML.cpp

 //===- SourceCoverageViewHTML.cpp - A html code coverage view -------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file This file implements the html coverage renderer.
 ///
 //===----------------------------------------------------------------------===//
 
-#include "CoverageReport.h"
 #include "SourceCoverageViewHTML.h"
+#include "CoverageReport.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/ThreadPool.h"
 #include <optional>
 
 using namespace llvm;
 
 namespace {
 
 // Return a string with the special characters in \p Str escaped.
 std::string escape(StringRef Str, const CoverageViewOptions &Opts) {

 // Create an anchor to \p Link with the label \p Str.
 std::string a(const std::string &Link, const std::string &Str,
               const std::string &TargetName = "") {
   std::string Name = TargetName.empty() ? "" : ("name='" + TargetName + "' ");
   return "<a " + Name + "href='" + Link + "'>" + Str + "</a>";
 }
 
 const char *BeginHeader =
     "<head>"
     "<meta name='viewport' content='width=device-width,initial-scale=1'>"
     "<meta charset='UTF-8'>";
 
 const char *CSSForCoverage =
     R"(.red {
   background-color: #ffd0d0;
 }
 .cyan {
if (PathToStyle.empty())
     OS << "<style>" << CSSForCoverage << "</style>";
   else
     OS << "<link rel='stylesheet' type='text/css' href='"
        << escape(PathToStyle, Opts) << "'>";
 
   OS << EndHeader << "<body>";
 }
 
+void emitTableRow(raw_ostream &OS, const CoverageViewOptions &Opts,
+                  const std::string &FirstCol, const FileCoverageSummary &FCS,
+                  bool IsTotals) {
+  SmallVector<std::string, 8> Columns;
+
+  // Format a coverage triple and add the result to the list of columns.
+  auto AddCoverageTripleToColumn =
+      [&Columns, &Opts](unsigned Hit, unsigned Total, float Pctg) {
+        std::string S;
+        {
+          raw_string_ostream RSO{S};
+          if (Total)
+            RSO << format("%*.2f", 7, Pctg) << "% ";
+          else
+            RSO << "- ";
+          RSO << '(' << Hit << '/' << Total << ')';
+        }
+        const char *CellClass = "column-entry-yellow";
+        if (Pctg >= Opts.HighCovWatermark)
+          CellClass = "column-entry-green";
+        else if (Pctg < Opts.LowCovWatermark)
+          CellClass = "column-entry-red";
+        Columns.emplace_back(tag("td", tag("pre", S), CellClass));
+      };
+
+  Columns.emplace_back(tag("td", tag("pre", FirstCol)));
+  AddCoverageTripleToColumn(FCS.FunctionCoverage.getExecuted(),
+                            FCS.FunctionCoverage.getNumFunctions(),
+                            FCS.FunctionCoverage.getPercentCovered());
+  if (Opts.ShowInstantiationSummary)
+    AddCoverageTripleToColumn(FCS.InstantiationCoverage.getExecuted(),
+                              FCS.InstantiationCoverage.getNumFunctions(),
+                              FCS.InstantiationCoverage.getPercentCovered());
+  AddCoverageTripleToColumn(FCS.LineCoverage.getCovered(),
+                            FCS.LineCoverage.getNumLines(),
+                            FCS.LineCoverage.getPercentCovered());
+  if (Opts.ShowRegionSummary)
+    AddCoverageTripleToColumn(FCS.RegionCoverage.getCovered(),
+                              FCS.RegionCoverage.getNumRegions(),
+                              FCS.RegionCoverage.getPercentCovered());
+  if (Opts.ShowBranchSummary)
+    AddCoverageTripleToColumn(FCS.BranchCoverage.getCovered(),
+                              FCS.BranchCoverage.getNumBranches(),
+                              FCS.BranchCoverage.getPercentCovered());
+
+  if (IsTotals)
+    OS << tag("tr", join(Columns.begin(), Columns.end(), ""), "light-row-bold");
+  else
+    OS << tag("tr", join(Columns.begin(), Columns.end(), ""), "light-row");
+}
+
 void emitEpilog(raw_ostream &OS) {
   OS << "</body>"
      << "</html>";
 }
 
+/// Determine the length of the longest redundant prefix of the substrs starts
+/// from \p LCP in \p Paths. \p Paths can't be empty. If there's only one path
+/// in \p Paths, the length of the path is returned. Note this is differnet from
+/// the behavior for the function of the same name in CoverageReport.cpp .
+unsigned getRedundantPrefixLen(ArrayRef<StringRef> Paths, unsigned LCP = 0) {
+  assert(!Paths.empty() && "Paths must have at least one element");
+
+  auto Iter = Paths.begin();
+  auto IterE = Paths.end();
+  auto First = Iter->substr(LCP);
+  SmallVector<StringRef, 16> CommonComponents{sys::path::begin(First),
+                                              sys::path::end(First)};
+  unsigned Len = First.size() - LCP;
+  while (++Iter != IterE) {
+    auto Substr = Iter->substr(LCP);
+    auto I = sys::path::begin(Substr);
+    auto E = sys::path::end(Substr);
+
+    unsigned Index = 0;
+    while (I != E) {
+      if (Index >= CommonComponents.size())
+        break;
+      if (CommonComponents[Index] != *I) {
+        CommonComponents.resize(Index);
+        Len = I - sys::path::begin(Substr);
+        break;
+      }
+      ++Index, ++I;
+    }
+  }
+
+  return Len;
+}
+
 } // anonymous namespace
 
 Expected<CoveragePrinter::OwnedStream>
 CoveragePrinterHTML::createViewFile(StringRef Path, bool InToplevel) {
   auto OSOrErr = createOutputStream(Path, "html", InToplevel);
   if (!OSOrErr)
     return OSOrErr;
 
CoveragePrinterHTML::buildLinkToFile(StringRef SF,
   sys::path::remove_dots(LinkTextStr, /*remove_dot_dot=*/true);
   sys::path::native(LinkTextStr);
   std::string LinkText = escape(LinkTextStr, Opts);
   std::string LinkTarget =
       escape(getOutputPath(SF, "html", /*InToplevel=*/false), Opts);
   return a(LinkTarget, LinkText);
 }
 
+Error CoveragePrinterHTML::emitStyleSheet() {
+  auto CSSOrErr = createOutputStream("style", "css", /*InToplevel=*/true);
+  if (Error E = CSSOrErr.takeError())
+    return E;
+
+  OwnedStream CSS = std::move(CSSOrErr.get());
+  CSS->operator<<(CSSForCoverage);
+
+  return Error::success();
+}
+
+void CoveragePrinterHTML::emitReportHeader(raw_ostream &OSRef,
+                                           const std::string &Title) {
+  // Emit some basic information about the coverage report.
+  if (Opts.hasProjectTitle())
+    OSRef << tag(ProjectTitleTag, escape(Opts.ProjectTitle, Opts));
+  OSRef << tag(ReportTitleTag, Title);
+  if (Opts.hasCreatedTime())
+    OSRef << tag(CreatedTimeTag, escape(Opts.CreatedTimeStr, Opts));
+
+  // Emit a link to some documentation.
+  OSRef << tag("p", "Click " +
+                        a("http://clang.llvm.org/docs/"
+                          "SourceBasedCodeCoverage.html#interpreting-reports",
+                          "here") +
+                        " for information about interpreting this report.");
+
+  // Emit a table containing links to reports for each file in the covmapping.
+  // Exclude files which don't contain any regions.
+  OSRef << BeginCenteredDiv << BeginTable;
+  emitColumnLabelsForIndex(OSRef, Opts);
+}
+
 /// Render a file coverage summary (\p FCS) in a table row. If \p IsTotals is
 /// false, link the summary to \p SF.
 void CoveragePrinterHTML::emitFileSummary(raw_ostream &OS, StringRef SF,
                                           const FileCoverageSummary &FCS,
                                           bool IsTotals) const {
-  SmallVector<std::string, 8> Columns;
-
-  // Format a coverage triple and add the result to the list of columns.
-  auto AddCoverageTripleToColumn =
-      [&Columns, this](unsigned Hit, unsigned Total, float Pctg) {
-        std::string S;
-        {
-          raw_string_ostream RSO{S};
-          if (Total)
-            RSO << format("%*.2f", 7, Pctg) << "% ";
-          else
-            RSO << "- ";
-          RSO << '(' << Hit << '/' << Total << ')';
-        }
-        const char *CellClass = "column-entry-yellow";
-        if (Pctg >= Opts.HighCovWatermark)
-          CellClass = "column-entry-green";
-        else if (Pctg < Opts.LowCovWatermark)
-          CellClass = "column-entry-red";
-        Columns.emplace_back(tag("td", tag("pre", S), CellClass));
-      };
-
   // Simplify the display file path, and wrap it in a link if requested.
   std::string Filename;
   if (IsTotals) {
     Filename = std::string(SF);
   } else {
     Filename = buildLinkToFile(SF, FCS);
   }
 
-  Columns.emplace_back(tag("td", tag("pre", Filename)));
-  AddCoverageTripleToColumn(FCS.FunctionCoverage.getExecuted(),
-                            FCS.FunctionCoverage.getNumFunctions(),
-                            FCS.FunctionCoverage.getPercentCovered());
-  if (Opts.ShowInstantiationSummary)
-    AddCoverageTripleToColumn(FCS.InstantiationCoverage.getExecuted(),
-                              FCS.InstantiationCoverage.getNumFunctions(),
-                              FCS.InstantiationCoverage.getPercentCovered());
-  AddCoverageTripleToColumn(FCS.LineCoverage.getCovered(),
-                            FCS.LineCoverage.getNumLines(),
-                            FCS.LineCoverage.getPercentCovered());
-  if (Opts.ShowRegionSummary)
-    AddCoverageTripleToColumn(FCS.RegionCoverage.getCovered(),
-                              FCS.RegionCoverage.getNumRegions(),
-                              FCS.RegionCoverage.getPercentCovered());
-  if (Opts.ShowBranchSummary)
-    AddCoverageTripleToColumn(FCS.BranchCoverage.getCovered(),
-                              FCS.BranchCoverage.getNumBranches(),
-                              FCS.BranchCoverage.getPercentCovered());
-
-  if (IsTotals)
-    OS << tag("tr", join(Columns.begin(), Columns.end(), ""), "light-row-bold");
-  else
-    OS << tag("tr", join(Columns.begin(), Columns.end(), ""), "light-row");
+  emitTableRow(OS, Opts, Filename, FCS, IsTotals);
 }
 
 Error CoveragePrinterHTML::createIndexFile(
     ArrayRef<std::string> SourceFiles, const CoverageMapping &Coverage,
     const CoverageFiltersMatchAll &Filters) {
   // Emit the default stylesheet.
-  auto CSSOrErr = createOutputStream("style", "css", /*InToplevel=*/true);
-  if (Error E = CSSOrErr.takeError())
+  if (Error E = emitStyleSheet())
     return E;
 
-  OwnedStream CSS = std::move(CSSOrErr.get());
-  CSS->operator<<(CSSForCoverage);
-
   // Emit a file index along with some coverage statistics.
   auto OSOrErr = createOutputStream("index", "html", /*InToplevel=*/true);
   if (Error E = OSOrErr.takeError())
     return E;
   auto OS = std::move(OSOrErr.get());
   raw_ostream &OSRef = *OS.get();
 
   assert(Opts.hasOutputDirectory() && "No output directory for index file");
   emitPrelude(OSRef, Opts, getPathToStyle(""));
 
-  // Emit some basic information about the coverage report.
-  if (Opts.hasProjectTitle())
-    OSRef << tag(ProjectTitleTag, escape(Opts.ProjectTitle, Opts));
-  OSRef << tag(ReportTitleTag, "Coverage Report");
-  if (Opts.hasCreatedTime())
-    OSRef << tag(CreatedTimeTag, escape(Opts.CreatedTimeStr, Opts));
-
-  // Emit a link to some documentation.
-  OSRef << tag("p", "Click " +
-                        a("http://clang.llvm.org/docs/"
-                          "SourceBasedCodeCoverage.html#interpreting-reports",
-                          "here") +
-                        " for information about interpreting this report.");
+  emitReportHeader(OSRef, "Coverage Report");
 
-  // Emit a table containing links to reports for each file in the covmapping.
-  // Exclude files which don't contain any regions.
-  OSRef << BeginCenteredDiv << BeginTable;
-  emitColumnLabelsForIndex(OSRef, Opts);
   FileCoverageSummary Totals("TOTALS");
   auto FileReports = CoverageReport::prepareFileReports(
       Coverage, Totals, SourceFiles, Opts, Filters);
   bool EmptyFiles = false;
   for (unsigned I = 0, E = FileReports.size(); I < E; ++I) {
     if (FileReports[I].FunctionCoverage.getNumFunctions())
       emitFileSummary(OSRef, SourceFiles[I], FileReports[I]);
     else
       EmptyFiles = true;
   }
   emitFileSummary(OSRef, "Totals", Totals, /*IsTotals=*/true);
   OSRef << EndTable << EndCenteredDiv;
 
   // Emit links to files which don't contain any functions. These are normally
   // not very useful, but could be relevant for code which abuses the
   // preprocessor.
   if (EmptyFiles && Filters.empty()) {
     OSRef << tag("p", "Files which contain no functions. (These "
                       "files contain code pulled into other files "
                       "by the preprocessor.)\n");
     OSRef << BeginCenteredDiv << BeginTable;
     for (unsigned I = 0, E = FileReports.size(); I < E; ++I)
       if (!FileReports[I].FunctionCoverage.getNumFunctions()) {
         std::string Link = buildLinkToFile(SourceFiles[I], FileReports[I]);
         OSRef << tag("tr", tag("td", tag("pre", Link)), "light-row") << '\n';
       }
     OSRef << EndTable << EndCenteredDiv;
   }
 
   OSRef << tag("h5", escape(Opts.getLLVMVersionString(), Opts));
   emitEpilog(OSRef);
 
   return Error::success();
 }
 
+Error CoveragePrinterHTMLDirectory::createIndexFile(
+    ArrayRef<std::string> SourceFiles, const CoverageMapping &Coverage,
+    const CoverageFiltersMatchAll &Filters) {
+  // The createSubIndexFile function only works when SourceFiles is
+  // not empty. So we fallback to CoveragePrinterHTML when it is.
+  if (SourceFiles.empty())
+    return CoveragePrinterHTML::createIndexFile(SourceFiles, Coverage, Filters);
+
+  // Emit the default stylesheet.
+  if (Error E = emitStyleSheet())
+    return E;
+
+  // Emit index files in every subdirectory.
+  std::vector<StringRef> Files(SourceFiles.begin(), SourceFiles.end());
+  ThreadPoolStrategy PoolS = hardware_concurrency(Opts.NumThreads);
+  if (Opts.NumThreads == 0) {
+    PoolS = heavyweight_hardware_concurrency(Files.size());
+    PoolS.Limit = true;
+  }
+  ThreadPool Pool(PoolS); // The above initialization code was copied from
+                          // CoverageReport::prepareFileReports.
+
+  unsigned RootLCP;
+  if (SourceFiles.size() == 1)
+    RootLCP = sys::path::parent_path(SourceFiles.front()).size() + 1;
+  // We handle this case specially. If there's only one file in total,
+  // its parent directory is considered as the LCP.
+  else
+    RootLCP = getRedundantPrefixLen(Files);
+  auto LCPath = SourceFiles.front().substr(0, RootLCP);
+
+  CoverageNow = &Coverage;
+  FiltersNow = &Filters;
+  PoolNow = &Pool;
+  LCPStack = {RootLCP};
+  FileCoverageSummary RootTotal({}); // The root total is useless and ignored.
+  if (auto E = createSubIndexFile(Files, &RootTotal))
+    return E;
+
+  // Emit the top level index file. Top level index file is just a redirection
+  // to the index file in the LCP directory.
+  auto OSOrErr = createOutputStream("index", "html", /*InToplevel=*/true);
+  if (auto E = OSOrErr.takeError())
+    return E;
+  auto OS = std::move(OSOrErr.get());
+  auto LCPIndexFilePath =
+      getOutputPath(LCPath + "index", "html", /*InToplevel=*/false);
+  *OS.get() << R"(<!DOCTYPE html>
+  <html>
+    <head>
+      <meta http-equiv="Refresh" content="0; url=')"
+            << LCPIndexFilePath << R"('" />
+    </head>
+    <body></body>
+  </html>
+  )";
+
+  return Error::success();
+}
+
+/// Filter out files in LCP directory, group others by subdirectory and call
+/// recursively on them. The size of \p Files must be greater than 0. The
+/// FileCoverageSummary of this directory will be added to \p Totals.
+/// Current LCP is LCPStack.back().
+Error CoveragePrinterHTMLDirectory::createSubIndexFile(
+    const ArrayRef<StringRef> &Files, FileCoverageSummary *Totals) {
+  assert(!Files.empty() && "Only works when Files is not empty");
+  auto LCP = LCPStack.back();
+  auto LCPath = Files.front().substr(0, LCP).str();
+  assert(LCPath.back() == '/' && "LCPath should end with a slash");
+  auto OSOrErr =
+      createOutputStream(LCPath + "index", "html", /*InToplevel=*/false);
+  if (auto E = OSOrErr.takeError())
+    return E;
+  auto OS = std::move(OSOrErr.get());
+  raw_ostream &OSRef = *OS.get();
+
+  assert(Opts.hasOutputDirectory() && "No output directory for index file");
+  auto IndexHtmlPath =
+      getOutputPath(LCPath + "index", "html", /*InToplevel=*/false);
+  emitPrelude(OSRef, Opts, getPathToStyle(IndexHtmlPath));
+
+  auto NavLink = buildTitleLinks(LCPath);
+  emitReportHeader(OSRef, "Coverage Report (" + NavLink + ")");
+
+  // Filter out files in current directory and group others by subdiretory.
+  // Use ordered map to keep entries in order.
+  StringMap<FileCoverageSummary> CurrentFiles;
+  StringMap<SmallVector<StringRef, 0>> CurrentDirs;
+  for (auto &&File : Files) {
+    auto SubPath = File.substr(LCPath.size());
+
+    auto I = sys::path::begin(SubPath);
+    auto E = sys::path::end(SubPath);
+    assert(I != E && "Such case should have been filtered out in the caller");
+
+    auto Name = *I;
+    if (++I == E) {
+      auto Iter = CurrentFiles.insert_or_assign(Name, File).first;
+      PoolNow->async(&CoverageReport::prepareSingleFileReport, File,
+                     CoverageNow, Opts, LCP, &Iter->second, FiltersNow);
+    } else {
+      CurrentDirs[Name].push_back(File);
+    }
+  }
+
+  // Emit subdirectories first to make them at the top of the table.
+  FileCoverageSummary CurrentTotals(LCPath);
+  for (auto &&Dir : CurrentDirs) {
+    if (Dir.second.size() == 1) {
+      // If there's only one file in that subdirectory, we don't bother to
+      // create index file for it. Instead, we show the file report directly at
+      // this level.
+      auto RelPath = Dir.second.front().substr(LCP);
+      FileCoverageSummary FCS(RelPath);
+      CoverageReport::prepareSingleFileReport(Dir.second[0], CoverageNow, Opts,
+                                              LCP, &FCS, FiltersNow);
+      emitTableRow(OSRef, Opts, buildRelLinkToFile(RelPath, /*IsDir=*/false),
+                   FCS, /*IsTotals=*/false);
+      CurrentTotals += FCS;
+    } else {
+      auto SubDirLCP = getRedundantPrefixLen(Dir.second, LCP);
+      auto RelPath = Dir.second.front().substr(LCP, SubDirLCP);
+      FileCoverageSummary FCS(RelPath);
+      LCPStack.push_back(LCP + SubDirLCP);
+      if (auto E = createSubIndexFile(Dir.second, &FCS))
+        return E;
+      emitTableRow(OSRef, Opts, buildRelLinkToFile(RelPath, /*IsDir=*/true),
+                   FCS, /*IsTotals=*/false);
+      CurrentTotals += FCS;
+    }
+  }
+
+  // Then emit the files in current directory.
+  PoolNow->wait(); // Makes files reporting overlap with subdir reporting.
+  std::vector<decltype(CurrentFiles)::const_iterator> EmptyFiles;
+  for (auto I = CurrentFiles.begin(), E = CurrentFiles.end(); I != E; ++I) {
+    if (I->second.FunctionCoverage.getNumFunctions())
+      emitTableRow(OSRef, Opts, buildRelLinkToFile(I->first(), /*IsDir=*/false),
+                   I->second, /*IsTotals=*/false);
+    else
+      EmptyFiles.push_back(I);
+    CurrentTotals += I->second;
+  }
+
+  // Emit the totals row.
+  emitTableRow(OSRef, Opts, "Totals", CurrentTotals, /*IsTotals=*/false);
+  OSRef << EndTable << EndCenteredDiv;
+
+  // Emit links to files which don't contain any functions. These are normally
+  // not very useful, but could be relevant for code which abuses the
+  // preprocessor.
+  if (!EmptyFiles.empty() && FiltersNow->empty()) {
+    OSRef << tag("p", "Files which contain no functions. (These "
+                      "files contain code pulled into other files "
+                      "by the preprocessor.)\n");
+    OSRef << BeginCenteredDiv << BeginTable;
+    for (auto I : EmptyFiles) {
+      auto Link = buildRelLinkToFile(I->first(), /*IsDir=*/false);
+      OSRef << tag("tr", tag("td", tag("pre", Link)), "light-row") << '\n';
+    }
+    OSRef << EndTable << EndCenteredDiv;
+  }
+
+  // Emit epilog.
+  OSRef << tag("h5", escape(Opts.getLLVMVersionString(), Opts));
+  emitEpilog(OSRef);
+
+  *Totals += CurrentTotals;
+  LCPStack.pop_back();
+  return Error::success();
+}
+
+/// Make a title with hyperlinks to the index.html files of each hierarchy
+/// of the report.
+std::string
+CoveragePrinterHTMLDirectory::buildTitleLinks(StringRef LCPath) const {
+  // For each report level in LCPStack, extract the path component and
+  // calculate the number of "../" relative to current LCPath.
+  SmallVector<std::pair<std::string, unsigned>, 16> Components;
+  auto Iter = LCPStack.begin(), IterE = LCPStack.end();
+  if (*Iter > 0)
+    Components.push_back({
+        LCPath.substr(0, *Iter - 1).str(), // Remove trailing slash.
+        0,
+    });
+  for (auto Last = *Iter; ++Iter != IterE; Last = *Iter) {
+    auto SubPath = LCPath.substr(Last, *Iter - Last - 1);
+    auto Level = unsigned(SubPath.count(sys::path::get_separator())) + 1;
+    Components.back().second += Level;
+    Components.emplace_back(SubPath, Level);
+  }
+
+  // Then we make the title accroding to Components.
+  std::string S;
+  for (auto I = Components.begin(), E = Components.end();;) {
+    auto &Name = I->first;
+    if (++I == E) {
+      S += a("./index.html", Name);
+      S += sys::path::get_separator();
+      break;
+    }
+
+    std::string Link;
+    for (unsigned J = I->second; J > 0; --J) {
+      Link += "..";
+      Link += sys::path::get_separator();
+    }
+    Link += "index.html";
+    S += a(Link, Name);
+    S += sys::path::get_separator();
+  }
+  return S;
+}
+
+std::string CoveragePrinterHTMLDirectory::buildRelLinkToFile(StringRef RelPath,
+                                                             bool IsDir) const {
+  SmallString<128> LinkTextStr(RelPath);
+  sys::path::remove_dots(LinkTextStr, /*remove_dot_dot=*/true);
+  sys::path::native(LinkTextStr);
+
+  std::string LinkTargetStr;
+  if (IsDir) {
+    LinkTextStr += sys::path::get_separator();
+    LinkTargetStr = (RelPath + "index.html").str();
+  } else {
+    LinkTargetStr = (RelPath + ".html").str();
+  }
+
+  auto LinkText = escape(LinkTextStr, Opts);
+  auto LinkTarget = escape(LinkTargetStr, Opts);
+  return a(LinkTarget, LinkText);
+}
+
 void SourceCoverageViewHTML::renderViewHeader(raw_ostream &OS) {
   OS << BeginCenteredDiv << BeginTable;
 }
 
 void SourceCoverageViewHTML::renderViewFooter(raw_ostream &OS) {
   OS << EndTable << EndCenteredDiv;
 }
 
for (unsigned I = 0, E = Segments.size() - 1; I < E; ++I) {
       const auto *CurSeg = Segments[I];
       if (!CurSeg->IsRegionEntry)
         continue;
       if (CurSeg->Count == LCS.getExecutionCount())
         continue;
 
       Snippets[I + 1] =
           tag("div", Snippets[I + 1] + tag("span", formatCount(CurSeg->Count),
                                            "tooltip-content"),
               "tooltip");
 
       if (getOptions().Debug)
         errs() << "Marker at " << CurSeg->Line << ":" << CurSeg->Col << " = "
                << formatCount(CurSeg->Count) << "\n";
     }
   }
 

llvm/utils/prepare-code-coverage-artifact.py

invocation = (
         + [
             "-format",
             "html",
             "-instr-profile",
             profile,
             "-o",
             report_dir,
             "-show-line-counts-or-regions",
+            "-show-directory-coverage",
             "-Xdemangler",
             "c++filt",
             "-Xdemangler",
             "-n",
         ]
         + restricted_dirs
     )
     if compilation_dir: