[x86-64] The '-fdelete-null-pointer-checks' pass not removed redundant code

[IshwaraK]

The clang compiler is not optimizing away pointer check in the foo() function in this code. The gcc removes that part of code as dead code after '-fdelete-null-pointer-checks' pass. We can observe these behaviors in platforms like X86, ARM.

The clang assembly code in x86-64; https://godbolt.org/z/nncroxMx4

The gcc assembly code in x86-64; https://godbolt.org/z/YbqPYGon4

#include <stdio.h>
#include <string.h>

char dest[50];

void foo(char* dest, char* src) {
  memcpy(dest, src, strlen(src));
  if (!src) { // <------- This is redundant check as pointer is already de-referenced
     printf("checking -fdelete-null-pointer-checks compiler optimization\n");
     printf("This source code is redundant\n");
     __builtin_abort ();
  }
  printf("After check for null pointer\n");
}

int main ()
{
   char src[50] = "-fdelete-null-pointer-checks";
   foo(dest, src);
   return 0;
}

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Ishwara Sharma (IshwaraK)

The clang compiler is not optimizing away pointer check in the foo() function in this code. The gcc removes that part of code as dead code after '-fdelete-null-pointer-checks' pass. We can observe these behaviors in platforms like X86, ARM.

The clang assembly code in x86-64; https://godbolt.org/z/nncroxMx4

The gcc assembly code in x86-64; https://godbolt.org/z/YbqPYGon4

#include <stdio.h>
#include <string.h>

char dest[50];

void foo(char* dest, char* src) {
  memcpy(dest, src, strlen(src));
  if (!src) { // <------- This is redundant check as pointer is already de-referenced
     printf("checking -fdelete-null-pointer-checks compiler optimization\n");
     printf("This source code is redundant\n");
     __builtin_abort ();
  }
  printf("After check for null pointer\n");
}

int main ()
{
   char src[50] = "-fdelete-null-pointer-checks";
   foo(dest, src);
   return 0;
}

[nikic]

This optimization is no longer legal since https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3322.pdf. It's only valid if the length passed to memcpy is known non-zero, in which case Clang does perform the optimization.

[dtcxzyw]

This optimization is no longer legal since https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3322.pdf. It's only valid if the length passed to memcpy is known non-zero, in which case Clang does perform the optimization.

But strlen(src) implies that src is nonnull.

[nikic]

Oh sorry, I thought this issue was about the memcpy call, not the strlen call. I agree that this optimization is valid for strlen. It look like we don't infer nonnull for strlen in BuildLibCalls.