[llvm-bugs] [Bug 50120] New: DSE fails to shorten sizes of memsets

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50120

            Bug ID: 50120
           Summary: DSE fails to shorten sizes of memsets
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

https://godbolt.org/z/48TT9dPvP

#include <string.h>

struct pam {
  void *p1;
  void *p2;
  #ifdef LONG
  unsigned long size;
  #else
  unsigned int size;
  unsigned int pad;
  #endif
};

extern int use(struct pam *param);

unsigned int foo(void) {
  struct pam s_pam;
  memset(&s_pam, 0, sizeof(struct pam));
  s_pam.size = 1;
  return use(&s_pam);
}



For LONG case DSE changes 24->16
  call void @llvm.memset.p0i8.i64(i8* noundef nonnull align 8
dereferenceable(24) %2, i8 0, i64 16, i1 false), !dbg !24
For INT case - no change - 24->24 
  call void @llvm.memset.p0i8.i64(i8* noundef nonnull align 8
dereferenceable(24) %2, i8 0, i64 24, i1 false), !dbg !23


INT case:
foo(): # @foo()
  sub rsp, 24
  xorps xmm0, xmm0
  movaps xmmword ptr [rsp], xmm0
  mov qword ptr [rsp + 16], 0        // ******
  mov dword ptr [rsp + 16], 1        // ******
  mov rdi, rsp
  call use(pam*)
  add rsp, 24
  ret

LONG case:
foo(): # @foo()
  sub rsp, 24
  xorps xmm0, xmm0
  movaps xmmword ptr [rsp], xmm0
  mov qword ptr [rsp + 16], 1        // ******
  mov rdi, rsp
  call use(pam*)
  add rsp, 24
  ret

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