[PATCH] D130956: [X86][MC] Always emit `rep` prefix for `bsf`

Wed Aug 3 14:44:21 PDT 2022

craig.topper added a comment.

In D130956#3697844 <https://reviews.llvm.org/D130956#3697844>, @fmayer wrote:

> This seems related to an error we see on our sanitizer bots: https://lab.llvm.org/buildbot/#/builders/37/builds/15446/steps/33/logs/stdio
>
>   FAIL: Builtins-i386-linux :: ffssi2_test.c (2860 of 8387)
>   ******************** TEST 'Builtins-i386-linux :: ffssi2_test.c' FAILED ********************
>   Script:
>   --
>   : 'RUN: at line 1';       /b/sanitizer-x86_64-linux/build/llvm_build64/bin/clang   -gline-tables-only  -m32 -DCOMPILER_RT_HAS_FLOAT16  -fno-builtin -I /b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/lib/builtins -nodefaultlibs /b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/test/builtins/Unit/ffssi2_test.c /b/sanitizer-x86_64-linux/build/compiler_rt_build/lib/linux/libclang_rt.builtins-i386.a -lc -lm -o /b/sanitizer-x86_64-linux/build/compiler_rt_build/test/builtins/Unit/I386LinuxConfig/Output/ffssi2_test.c.tmp &&  /b/sanitizer-x86_64-linux/build/compiler_rt_build/test/builtins/Unit/I386LinuxConfig/Output/ffssi2_test.c.tmp
>   --
>   Exit Code: 1
>   Command Output (stdout):
>   --
>   error in __ffssi2(0x0) = 33, expected 0
>   --
>   ********************
>   Testing:  0.. 10.. 20.. 30.
>   FAIL: Builtins-x86_64-linux :: ffssi2_test.c (3084 of 8387)
>   ******************** TEST 'Builtins-x86_64-linux :: ffssi2_test.c' FAILED ********************
>   Script:
>   --
>   : 'RUN: at line 1';       /b/sanitizer-x86_64-linux/build/llvm_build64/bin/clang   -gline-tables-only  -m64 -DCOMPILER_RT_HAS_FLOAT16  -fno-builtin -I /b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/lib/builtins -nodefaultlibs /b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/test/builtins/Unit/ffssi2_test.c /b/sanitizer-x86_64-linux/build/compiler_rt_build/lib/linux/libclang_rt.builtins-x86_64.a -lc -lm -o /b/sanitizer-x86_64-linux/build/compiler_rt_build/test/builtins/Unit/X86_64LinuxConfig/Output/ffssi2_test.c.tmp &&  /b/sanitizer-x86_64-linux/build/compiler_rt_build/test/builtins/Unit/X86_64LinuxConfig/Output/ffssi2_test.c.tmp
>   --
>   Exit Code: 1
>   Command Output (stdout):
>   --
>   error in __ffssi2(0x0) = 33, expected 0
>   --
>   ********************
>   Testing:  0.. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..

Random guess, we forgot to take into account that tzcnt and bsf don't set the flags the same way. If the flags from the bsf are being used we can't put an f3 in front of it.

Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D130956