Don't append the working directory to absolute paths

[aprantl]

Summary

This fixes a bug that happens when using -fdebug-prefix-map to remap an absolute path to a relative path. Since the path was absolute before remapping, it is safe to assume that concatenating the remapped working directory would be wrong.

[dblaikie]

@thakis - I think you folks over in Chrome land implemented/use -fdebug-prefix-map (or do you use -fdebug-compilation-dir?) so might be interested in this change.

[abrachet]

We're seeing test failures in https://ci.chromium.org/ui/p/fuchsia/builders/toolchain.ci/clang-linux-x64/b8821217427732511185/overview

[thakis]

Breaks tests on Linux and Mac, e.g. http://45.33.8.238/linux/69643/step_7.txt and http://45.33.8.238/win/53984/step_7.txt

Please take a look and revert for now if it takes a while to fix.

[thakis]

In D113718#3208927, @dblaikie wrote:

@thakis - I think you folks over in Chrome land implemented/use -fdebug-prefix-map (or do you use -fdebug-compilation-dir?) so might be interested in this change.

We're using fdebug-compilation-dir. With fdebug-prefix-map, your command line flags are machine-specific, which makes builds not universally deterministic (per https://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html)

clang/lib/CodeGen/CGDebugInfo.cpp

if (std::distance(llvm::sys::path::begin(CurDir), CurDirIt) == 1) {
       File = RemappedFile;
     } else {
       for (; FileIt != FileE; ++FileIt)
         llvm::sys::path::append(FileBuf, *FileIt);
       Dir = DirBuf;
       File = FileBuf;
     }
   } else {
+    if (!llvm::sys::path::is_absolute(FileName))
       Dir = CurDir;
     File = RemappedFile;
   }
   llvm::DIFile *F = DBuilder.createFile(File, Dir, CSInfo, Source);
   DIFileCache[FileName.data()].reset(F);
   return F;
 }
 
 std::string CGDebugInfo::remapDIPath(StringRef Path) const {

clang/test/CodeGen/debug-prefix-map.c

 // RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - | FileCheck %s -check-prefix CHECK-NO-MAIN-FILE-NAME
 // RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH=empty %s -emit-llvm -o - | FileCheck %s -check-prefix CHECK-EVIL
 // RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - -main-file-name debug-prefix-map.c | FileCheck %s
 // RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - -fdebug-compilation-dir %p | FileCheck %s -check-prefix CHECK-COMPILATION-DIR
 // RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - -isysroot %p -debugger-tuning=lldb | FileCheck %s -check-prefix CHECK-SYSROOT
 // RUN: %clang -g -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s
 // RUN: %clang -g -ffile-prefix-map=%p=/UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang -g -fdebug-prefix-map=%p=./UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s --check-prefix=CHECK-REL
+// RUN: %clang -g -ffile-prefix-map=%p=./UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s --check-prefix=CHECK-REL
 
 #include "Inputs/stdio.h"
 
 int main(int argc, char **argv) {
   (void)argc;
   (void)argv;
   return 0;
 }

 // CHECK: !DIFile(filename: "/UNLIKELY_PATH/empty{{/|\\\\}}{{.*}}Inputs/stdio.h",
 // CHECK-SAME:    directory: "{{()|(.*:.*)}}")
 // CHECK-NOT: !DIFile(filename:
 
 // CHECK-COMPILATION-DIR: !DIFile(filename: "{{.*}}", directory: "/UNLIKELY_PATH/empty")
 // CHECK-COMPILATION-DIR: !DIFile(filename: "{{.*}}Inputs/stdio.h", directory: "/UNLIKELY_PATH/empty")
 // CHECK-COMPILATION-DIR-NOT: !DIFile(filename:
 // CHECK-SYSROOT: !DICompileUnit({{.*}}sysroot: "/UNLIKELY_PATH/empty"
+
+// CHECK-REL: !DIFile(filename: "./UNLIKELY_PATH/empty{{/|\\\\}}{{.*}}",
+// CHECK-REL: !DIFile(filename: "./UNLIKELY_PATH/empty{{/|\\\\}}{{.*}}Inputs/stdio.h",
+// CHECK-REL-SAME:    directory: "")