[llvm-bugs] [Bug 33914] New: Performance degradation of memcmp(size=24) after commit 308322

via llvm-bugs llvm-bugs at lists.llvm.org
Mon Jul 24 12:40:04 PDT 2017


            Bug ID: 33914
           Summary: Performance degradation of memcmp(size=24) after
                    commit 308322
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: zvi.rackover at intel.com
                CC: llvm-bugs at lists.llvm.org

We observed a 13% degradation in an internal benchmark after commit 308322.

The minimal reproducer:

  define i32 @foo(i8* %A, i8* %B) {
     %res = call i32 @memcmp(i8* %A, i8* %B, i64 24)
     ret i32 %res
  declare i32 @memcmp(i8* nocapture, i8* nocapture, i64) local_unnamed_addr #5

Before the commit the call to memcmp was lowered to a call to glibc's memcmp
which was dispatched to __memcmp_sse4_1. The hot code in __memcmp_sse4_1 was
doing 1-XMM-load pair+ptest+jcc+8-byte-load-pair+cmp+jcc:

       │        movdqu -0x18(%rdi),%xmm2                                        
       │        movdqu -0x18(%rsi),%xmm1 
 11.11 │        pxor   %xmm1,%xmm2                                              
  7.41 │        ptest  %xmm2,%xmm0                                              
 14.81 │      ↓ jae    15e8                                                     
 14.81 │        mov    -0x8(%rsi),%rcx                                          
       │        mov    -0x8(%rdi),%rax                                          
       │        cmp    %rax,%rcx                                                
       │      ↓ jne    1603                                                     
       │        xor    %eax,%eax                                                
 11.11 │      ← retq

After the commit the memcmp is expanded inline to three

# # BB#0:                                 # %loadbb
        movbeq  (%rdi), %rcx
        movbeq  (%rsi), %rdx
        cmpq    %rdx, %rcx
        jne     .LBB0_1
# BB#2:                                 # %loadbb1
        movbeq  8(%rdi), %rcx
        movbeq  8(%rsi), %rdx
        cmpq    %rdx, %rcx
        jne     .LBB0_1
# BB#3:                                 # %loadbb2
        movbeq  16(%rdi), %rcx
        movbeq  16(%rsi), %rdx
        xorl    %eax, %eax
        cmpq    %rdx, %rcx
        jne     .LBB0_1
# BB#4:                                 # %endblock
.LBB0_1:                                # %res_block
        cmpq    %rdx, %rcx
        movl    $-1, %ecx
        movl    $1, %eax
        cmovbl  %ecx, %eax

Options for fixing:

1. Improve the inline expansion to generate a similar sequence to glibc's: 1
16-byte pair load + ptest + jcc + 8-byte load + cmp + jmp

2. call libc's memcmp

I would like to request this commit be reverted until we get this issue fixed.

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