[PATCH] D134394: Update docs about [[likely]] vs. PGO

[serge via Phabricator via cfe-commits]

serge-sans-paille added a comment.

While running this small experiment: https://sergesanspaille.fedorapeople.org/proftest.sh
I get the following output:

  clang version 16.0.0 (...)
  Target: x86_64-unknown-linux-gnu
  Thread model: posix
  InstalledDir: ...
  ==== - instrumentation + likely: puts first
  
  00000000000006d0 <main>:
   6d0:	50                   	push   %rax
   6d1:	83 ff 01             	cmp    $0x1,%edi
   6d4:	75 10                	jne    6e6 <main+0x16>
   6d6:	48 8d 3d a7 00 00 00 	lea    0xa7(%rip),%rdi        # 784 <_IO_stdin_used+0x4>
   6dd:	e8 ce fe ff ff       	callq  5b0 <puts at plt>
   6e2:	31 c0                	xor    %eax,%eax
   6e4:	59                   	pop    %rcx
   6e5:	c3                   	retq   
   6e6:	48 8b 06             	mov    (%rsi),%rax
   6e9:	48 8b 56 08          	mov    0x8(%rsi),%rdx
   6ed:	48 8d 3d 9b 00 00 00 	lea    0x9b(%rip),%rdi        # 78f <_IO_stdin_used+0xf>
   6f4:	48 89 c6             	mov    %rax,%rsi
   6f7:	31 c0                	xor    %eax,%eax
   6f9:	e8 92 fe ff ff       	callq  590 <printf at plt>
   6fe:	eb e2                	jmp    6e2 <main+0x12>
  
  0000000000000700 <__libc_csu_init>:
  
  ====  + instrumentation + likely: puts first
  
  00000000000005e0 <main>:
   5e0:	50                   	push   %rax
   5e1:	83 ff 01             	cmp    $0x1,%edi
   5e4:	75 10                	jne    5f6 <main+0x16>
   5e6:	48 8d 3d 97 01 00 00 	lea    0x197(%rip),%rdi        # 784 <_IO_stdin_used+0x4>
   5ed:	e8 be ff ff ff       	callq  5b0 <puts at plt>
   5f2:	31 c0                	xor    %eax,%eax
   5f4:	59                   	pop    %rcx
   5f5:	c3                   	retq   
   5f6:	48 8b 06             	mov    (%rsi),%rax
   5f9:	48 8b 56 08          	mov    0x8(%rsi),%rdx
   5fd:	48 8d 3d 8b 01 00 00 	lea    0x18b(%rip),%rdi        # 78f <_IO_stdin_used+0xf>
   604:	48 89 c6             	mov    %rax,%rsi
   607:	31 c0                	xor    %eax,%eax
   609:	e8 82 ff ff ff       	callq  590 <printf at plt>
   60e:	eb e2                	jmp    5f2 <main+0x12>
  
  0000000000000610 <_start>:
  ./b1.out 12
  ==== - instrumentation - likely: puts first
  
  00000000000006d0 <main>:
   6d0:	50                   	push   %rax
   6d1:	83 ff 01             	cmp    $0x1,%edi
   6d4:	75 0e                	jne    6e4 <main+0x14>
   6d6:	48 8d 3d a7 00 00 00 	lea    0xa7(%rip),%rdi        # 784 <_IO_stdin_used+0x4>
   6dd:	e8 ce fe ff ff       	callq  5b0 <puts at plt>
   6e2:	eb 18                	jmp    6fc <main+0x2c>
   6e4:	48 8b 06             	mov    (%rsi),%rax
   6e7:	48 8b 56 08          	mov    0x8(%rsi),%rdx
   6eb:	48 8d 3d 9d 00 00 00 	lea    0x9d(%rip),%rdi        # 78f <_IO_stdin_used+0xf>
   6f2:	48 89 c6             	mov    %rax,%rsi
   6f5:	31 c0                	xor    %eax,%eax
   6f7:	e8 94 fe ff ff       	callq  590 <printf at plt>
   6fc:	31 c0                	xor    %eax,%eax
   6fe:	59                   	pop    %rcx
   6ff:	c3                   	retq   
  
  0000000000000700 <__libc_csu_init>:
  
  ====  + instrumentation - likely: printf first
  
  00000000000005e0 <main>:
   5e0:	50                   	push   %rax
   5e1:	83 ff 01             	cmp    $0x1,%edi
   5e4:	74 1c                	je     602 <main+0x22>
   5e6:	48 8b 06             	mov    (%rsi),%rax
   5e9:	48 8b 56 08          	mov    0x8(%rsi),%rdx
   5ed:	48 8d 3d 9b 01 00 00 	lea    0x19b(%rip),%rdi        # 78f <_IO_stdin_used+0xf>
   5f4:	48 89 c6             	mov    %rax,%rsi
   5f7:	31 c0                	xor    %eax,%eax
   5f9:	e8 92 ff ff ff       	callq  590 <printf at plt>
   5fe:	31 c0                	xor    %eax,%eax
   600:	59                   	pop    %rcx
   601:	c3                   	retq   
   602:	48 8d 3d 7b 01 00 00 	lea    0x17b(%rip),%rdi        # 784 <_IO_stdin_used+0x4>
   609:	e8 a2 ff ff ff       	callq  5b0 <puts at plt>
   60e:	eb ee                	jmp    5fe <main+0x1e>
  
  0000000000000610 <_start>:

To me, it means that `[[likely]]` takes precedence over profile information...


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D134394/new/

https://reviews.llvm.org/D134394