[llvm-bugs] [Bug 27503] New: useless moves generated by memcpy() to union

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27503

            Bug ID: 27503
           Summary: useless moves generated by memcpy() to union
           Product: new-bugs
           Version: 3.8
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: jhaberman at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Test case:

--

#include <string.h>

typedef union {
  char ch;
  float fl;
  double dbl;
} u;

u f(const void *p, int type) {
  u v;
  if (type == 1) {
    memcpy(&v, p, 1);
  } else if (type <= 5) {
    memcpy(&v, p, 4);
  } else if (type <= 8) {
    memcpy(&v, p, 8);
  }
  return v;
}


--

On every platform/version I've tested, compiling with -O3 I get bad code like:

--

0000000000000000 <f>:
   0:    83 fe 01                 cmp    esi,0x1
   3:    75 0e                    jne    13 <f+0x13>
   5:    0f b6 07                 movzx  eax,BYTE PTR [rdi]
   8:    66 48 0f 6e c0           movq   xmm0,rax
   d:    66 48 0f 7e c0           movq   rax,xmm0
  12:    c3                       ret    
  13:    83 fe 05                 cmp    esi,0x5
  16:    7f 0d                    jg     25 <f+0x25>
  18:    8b 07                    mov    eax,DWORD PTR [rdi]
  1a:    66 48 0f 6e c0           movq   xmm0,rax
  1f:    66 48 0f 7e c0           movq   rax,xmm0
  24:    c3                       ret    
  25:    83 fe 08                 cmp    esi,0x8
  28:    7f 04                    jg     2e <f+0x2e>
  2a:    f2 0f 10 07              movsd  xmm0,QWORD PTR [rdi]
  2e:    66 48 0f 7e c0           movq   rax,xmm0
  33:    c3                       ret

--

Note the move to eax, then to xmm0, then back to rax.

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