[PATCH] D154725: [SimplifyLibCalls] Fold strcmp for short string literals

[Maksim Kita via Phabricator via llvm-commits]

kitaisreal added a comment.

For such program.

  #include <cstring>
  
  bool testFunction1_1(const char* c)
  {
      return strcmp(c, "0") == 0;
  }

GCC emits:

  	.type	_Z15testFunction1_1PKc, @function
  _Z15testFunction1_1PKc:
  .LFB27:
  	.cfi_startproc
  	endbr64
  	movzbl	(%rdi), %eax
  	subl	$48, %eax
  	jne	.L2
  	movzbl	1(%rdi), %eax
  .L2:
  	testl	%eax, %eax
  	sete	%al
  	ret
  	.cfi_endproc
  .LFE27:
  	.size	_Z15testFunction1_1PKc, .-_Z15testFunction1_1PKc
  	.p2align 4
  	.globl	_Z15testFunction1_2PKc
  	.type	_Z15testFunction1_2PKc, @function

After changes clang emits:

  	.type	_Z15testFunction1_1PKc, at function
  _Z15testFunction1_1PKc:                 # @_Z15testFunction1_1PKc
  	.cfi_startproc
  # %bb.0:                                # %entry
  	movsbl	(%rdi), %eax
  	addl	$-48, %eax
  	je	.LBB0_1
  # %bb.2:                                # %strcmp_fold_sub_join
  	testl	%eax, %eax
  	sete	%al
  	retq
  .LBB0_1:                                # %strcmp_fold_sub_is_zero
  	movsbl	1(%rdi), %eax
  	testl	%eax, %eax
  	sete	%al
  	retq
  .Lfunc_end0:
  	.size	_Z15testFunction1_1PKc, .Lfunc_end0-_Z15testFunction1_1PKc
  	.cfi_endproc



================
Comment at: llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp:612
   if (!HasStr1 && HasStr2) {
     if (canTransformToMemCmp(CI, Str1P, Len2, DL))
       return copyFlags(
----------------
efriedma wrote:
> nikic wrote:
> > This is the transform that implements what you're trying to do, but taking dereferenceability into account.
> https://github.com/llvm/llvm-project/issues/58003 is requesting that we generate control flow for small strings.  For example, transforming `strcmp(c, "x") == 0` to `c[0] == 'x' && c[1] == '\0'`.  This is distinct from transforming it to a memcmp.  (Of course, the current version of this patch doesn't implement that.)
I updated implementation for single character str2 constant. If current approach is valid I will implement it for two characters str2 constant as well.
I am not sure if current approach with changing control flow inside this optimization is valid and safe, because it seems that other SimplifyLibCalls optimizations does not change control flow.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D154725/new/

https://reviews.llvm.org/D154725