[LLVMbugs] [Bug 21749] New: Missing optimization

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21749

            Bug ID: 21749
           Summary: Missing optimization
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: eugeni.stepanov at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

long a, b, c;

void f(long x) {
  a = x ^ 0x1;
  b = x ^ 0x2;
  c = x;
}

On x86_64 with -O2, clang generates the following:
0000000000000000 <_Z1fl>:
   0:    48 89 f8                 mov    %rdi,%rax
   3:    48 83 f0 01              xor    $0x1,%rax
   7:    48 89 05 00 00 00 00     mov    %rax,0x0(%rip)        # e <_Z1fl+0xe>
   e:    48 89 f8                 mov    %rdi,%rax
  11:    48 83 f0 02              xor    $0x2,%rax
  15:    48 89 05 00 00 00 00     mov    %rax,0x0(%rip)        # 1c
<_Z1fl+0x1c>
  1c:    48 89 3d 00 00 00 00     mov    %rdi,0x0(%rip)        # 23
<_Z1fl+0x23>
  23:    c3                       retq   

This:
 mov %rdi,%rax
 xor $0x2,%rax
could be written as:
 xor $0x3, %rax

This costs 1.5% code size with MemorySanitizer instrumentation, but we have a
workaround.

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