Updating the TipsAndTricks.rst documentation of Polly related to bugpoint

[alexsusu]

Summary

Updating the TipsAndTricks.rst documentation of Polly related to bugpoint.
The previous version explained poorly, incorrectly bugpoint - maybe they were referring to an older, simpler version of bugpoint.
We describe here the version of bugpoint as checked out from the SVN repository on Oct 2017.

[bollu]

LGTM! Thanks, I didn't realize that the options could interact with bugs in llc.

[alexsusu]

(nothing serious)

[alexsusu]

(nothing serious, part 2)

[Meinersbur]

Thanks for improving our documentation!

TipsAndTricks.rst

18–20

Are you sure the additional flags -safe-auto, -compile-custom, -compile-command are needed? I never used them. AFAIU, -compile-custom will cause that -opt-command and -opt-args to be ignored.

Generally, there are two cases the distinguish: opt crashes or it does not. If opt crashes, llc should never be called since opt is a step before llc. If opt does not crash, bugpoint switches to another mode that tries to isolate a miscompile. I never actively used that mode.

Could you clarify which of the two modes this applies to?

[alexsusu]

In D38953#900086, @Meinersbur wrote:

Thanks for improving our documentation!

Are you sure the additional flags -safe-auto, -compile-custom, -compile-command are needed? I never used them. AFAIU, -compile-custom will cause that -opt-command and -opt-args to be ignored.

After triple-checking, -safe-auto is not needed. However, -compile-custom, -compile-command (and also -opt-command, although it overlaps with -compile-command - maybe this should be improved in bugpoint) are very useful - trying to remove any of them results in bugpoint no longer working properly from what I see.
So I UPDATED the documentation s.t. bugpoint has the minimal number of arguments.

Generally, there are two cases to distinguish: opt crashes or it does not. If opt crashes, llc should never be called since opt is a step before llc. If opt does not crash, bugpoint switches to another mode that tries to isolate a miscompile. I never actively used that mode.

From what I understand bugpoint uses normally llc, but to make it reduce a failing test in opt (as it it is the case for Polly) requires to use the arguments from the previous paragraph.

Could you clarify which of the two modes this applies to?

During the tests I did with bugpoint with the above mentioned arguments it never called llc, nor did bugpoint produce an object file - all I obtained were .bc LLVM IR bitcode files.

Alex

[Meinersbur]

Could you check whether this works for you:

assume.diff

diff --git a/lib/Support/ScopHelper.cpp b/lib/Support/ScopHelper.cpp
index ac501113..9fea5471 100644
--- a/lib/Support/ScopHelper.cpp
+++ b/lib/Support/ScopHelper.cpp
@@ -500,7 +500,9 @@ bool polly::isIgnoredIntrinsic(const Value *V) {
     case llvm::Intrinsic::ptr_annotation:
     case llvm::Intrinsic::annotation:
     case llvm::Intrinsic::donothing:
+      break;
     case llvm::Intrinsic::assume:
+      llvm_unreachable("crash here");
     // Some debug info intrinsics are supported/ignored.
     case llvm::Intrinsic::dbg_value:
     case llvm::Intrinsic::dbg_declare:

(patch With this patch)

assume.c

#include "stdio.h"

int main(int argc, char *argv[]) {
  printf("Hello World!");
  for (int i = 0; i < argc; i+=1) {
    __builtin_assume(0 > argc);
  }
  return 0;
}

and

assume.sh

#! /bin/sh

LLVMPATH=${HOME}/build/llvm/release/bin

cat $0
cat assume.c
${LLVMPATH}/clang assume.c -O0 -S -emit-llvm -Xclang -disable-O0-optnone -o assume.ll
${LLVMPATH}/opt assume.ll -mem2reg -polly-scops -analyze
${LLVMPATH}/bugpoint -opt-command=${LLVMPATH}/opt assume.ll -mem2reg -polly-scops

echo 
echo "The reduced file is:" 
${LLVMPATH}/opt bugpoint-reduced-simplified.bc -S

For this outputs:

$ ./assume.sh
[...]
Dumped core
Emitted bitcode to 'bugpoint-reduced-simplified.bc'

*** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -polly-scops

The reduced file is:
; ModuleID = 'bugpoint-reduced-simplified.bc'
source_filename = "bugpoint-output-b82a770.bc"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: noinline nounwind uwtable
define void @main() #0 {
entry:
  br label %for.cond

for.cond:                                         ; preds = %for.body, %entry
  br i1 false, label %for.body, label %for.end

for.body:                                         ; preds = %for.cond
  call void @llvm.assume(i1 undef)
  br label %for.cond

for.end:                                          ; preds = %for.cond
  ret void
}

; Function Attrs: nounwind
declare void @llvm.assume(i1) #1

attributes #0 = { noinline nounwind uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { nounwind }

!llvm.ident = !{!0}

!0 = !{!"clang version 6.0.0 (trunk 314509) (llvm/trunk 314516)"}

TipsAndTricks.rst

 ==================================================
 Tips and Tricks on using and contributing to Polly
 ==================================================
 
 Commiting to polly trunk
 ------------------------
     - `General reference to git-svn workflow <https://stackoverflow.com/questions/190431/is-git-svn-dcommit-after-merging-in-git-dangerous>`_
 
 
 Using bugpoint to track down errors in large files
 --------------------------------------------------
 
     If you know the ``opt`` invocation and have a large ``.ll`` file that causes
     an error, ``bugpoint`` allows one to reduce the size of test cases.
 
-    The general calling pattern is:
+    The general calling pattern of ``bugpoint`` to reduce a .ll file on which Polly crashes is:
 
-    - ``$ bugpoint <file.ll> <pass that causes the crash> -opt-args <opt option flags>``
+    - ``$ bugpoint <file.ll> -safe-auto -compile-custom -compile-command=<opt cmd line>``
+
+                   ``-opt-command=<opt file with path> -opt-args=<opt option flags>``
+
+
+    By doing so we avoid testing ``llc``, which is another component ``bugpoint`` can test, since it can give errors like: <<Initializing execution environment: bugpoint: [...] Assertion `!Name.empty() && "Must have a name!"' failed.>> .
+
+    We advise the user to give all these arguments: compile-command, opt-command, although they repeat the information.
 
     An example invocation is:
 
-    - ``$ bugpoint crash.ll -polly-codegen -opt-args  -polly-canonicalize -polly-process-unprofitable``
+    - ``$ clang -O3 -mllvm -disable-llvm-optzns -emit-llvm -S -o file_noopt.ll -g file.c``
+    - ``$ bugpoint file_noopt.ll -safe-auto  -compile-custom``
+
+                   ``-compile-command="/llvm/bin/opt -O3 -polly -S"``
+
+                   ``-opt-command=/llvm/bin/opt -opt-args="-polly -S"``
+
+
+    For more documentation on bugpoint, `Visit the LLVM manual <http://llvm.org/docs/Bugpoint.html>`_, the `Command Guide <https://llvm.org/docs/CommandGuide/bugpoint.html>`_ and also the `LLVM blog <http://blog.llvm.org/2015/11/reduce-your-testcases-with-bugpoint-and.html>`_ .
+
+    It is worth mentioning that bugpoint can take a long time to finish, for example 39 hours running on 1 core of a workstation.
+    Note that running a release build of LLVM reduces the total runtime to 5.22 hours.
+
+    Note that since the ``bugpoint`` tool can be a bit tricky to use with Polly, if you have problems you can best check the source code of the bugpoint tool.
+    For example, to avoid the error "Sorry, I can't automatically select a safe interpreter!" we had to get inspired directly from the source code file: https://github.com/llvm-mirror/llvm/blob/master/tools/bugpoint/ExecutionDriver.cpp in order to use a command line as indicated above, with arguments compile-command, opt-command, opt-args.
 
-    For more documentation on bugpoint, `Visit the LLVM manual <http://llvm.org/docs/Bugpoint.html>`_
 
 
 Understanding which pass makes a particular change
 --------------------------------------------------
 
     If you know that something like `opt -O3 -polly` makes a change, but you wish to
     isolate which pass makes a change, the steps are as follows: