[llvm-bugs] [Bug 28833] New: strlen (s) == 0 not folded when return type is casted to unsigned

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28833

            Bug ID: 28833
           Summary: strlen (s) == 0 not folded when return type is casted
                    to unsigned
           Product: libraries
           Version: 3.6
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Transformation Utilities
          Assignee: unassignedbugs at nondot.org
          Reporter: prathamesh.kulkarni at linaro.org
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Hi,
For the following test-case, strlen (s) == 0 is not folded to *s == 0.
_Bool f2(const char *s)
{
  unsigned len = __builtin_strlen (s);
  return len == 0;
}

Assembly with clang -O2 -S:
  .text
        .file   "test-2.c"
        .globl  f2
        .align  16, 0x90
        .type   f2, at function
f2:                                     # @f2
        .cfi_startproc
# BB#0:
        pushq   %rax
.Ltmp0:
        .cfi_def_cfa_offset 16
        callq   strlen
        testl   %eax, %eax
        sete    %al
        popq    %rdx
        retq
.Ltmp1:
        .size   f2, .Ltmp1-f2
        .cfi_endproc


        .ident  "Ubuntu clang version 3.6.2-1 (tags/RELEASE_362/final) (based
on LLVM 3.6.2)"
        .section        ".note.GNU-stack","", at progbits

However if type of len is changed from unsigned to unsigned long,
then the transform takes place.
Is there a reason why the transform is disabled if the return
value (unsigned long) is stored in a smaller type (unsigned) ?

Thanks,
Prathamesh

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