[tree-ssa-strlen] missing strlen optimization of the empty string

[xiangzhai]

Hi fellows,

As PR83190 mentioned:
Testcase:

$ cat a.c
int f (void)
{
  char a[] = "012\0";

  return __builtin_strlen (a + 3);   // GCC 8.x folded into 0, but GCC 5.x failed
}

int g (void)
{
  char a[] = "012\0";

  return __builtin_strlen (a + 4);   // not folded
}

GCC 5.5 failed to fold for f and g:

;; Function f (f, funcdef_no=0, decl_uid=1486, cgraph_uid=0, symbol_order=0)

f ()
{
  char a[5];
  long unsigned int _3;
  int _4;

  <bb 2>:
  a = "012";
  _3 = __builtin_strlen (&MEM[(void *)&a + 3B]);
  _4 = (int) _3;
  a ={v} {CLOBBER};
  return _4;

}



;; Function g (g, funcdef_no=1, decl_uid=1490, cgraph_uid=1, symbol_order=1)

g ()
{
  char a[5];
  long unsigned int _3;
  int _4;

  <bb 2>:
  a = "012";
  _3 = __builtin_strlen (&MEM[(void *)&a + 4B]);
  _4 = (int) _3;
  a ={v} {CLOBBER};
  return _4;

}

And GCC 8.x only failed to fold for g:

;; Function f (f, funcdef_no=0, decl_uid=1582, cgraph_uid=0, symbol_order=0)

f ()
{
  <bb 2> [local count: 1073741825]:
  return 0;

}



;; Function g (g, funcdef_no=1, decl_uid=1586, cgraph_uid=1, symbol_order=1)

g ()
{
  char a[5];
  long unsigned int _1;
  int _4;

  <bb 2> [local count: 1073741825]:
  a = "012";
  _1 = __builtin_strlen (&MEM[(void *)&a + 4B]);
  _4 = (int) _1;
  a ={v} {CLOBBER};
  return _4;

}

But LLVM 7.x works:

$ /home/xiangzhai/llvm-build/bin/clang -O2 -S -emit-llvm a.c
$ cat a.ll
; ModuleID = 'a.c'
source_filename = "a.c"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-redhat-linux"

; Function Attrs: norecurse nounwind readnone uwtable
define i32 @f() local_unnamed_addr #0 {
  ret i32 0 ; <-- folded
}

; Function Attrs: norecurse nounwind readnone uwtable
define i32 @g() local_unnamed_addr #0 {
  ret i32 0 ; <-- folded
}

attributes #0 = { norecurse nounwind readnone uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "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" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{!"LLVM China clang version 7.0.0 (git@github.com:llvm-mirror/clang.git 87bcdaa7f2311f57c35c18efc6cbf5a973a67de7) (git@github.com:llvm-mirror/llvm.git 4eeea16aaacd6134fd411abcdbab15b630f3302b) (based on LLVM 7.0.0svn)"}

Regards,
Leslie Zhai