incubator-lucy-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Nick Wellnhofer <wellnho...@aevum.de>
Subject Re: [lucy-dev] Optimization Bug On Mac OS X
Date Fri, 23 Mar 2012 09:50:47 GMT
On 23/03/2012 00:59, Logan Bell wrote:
> Nick,
>
> I went ahead and created a JIRA story with the original e-mail of this
> thread:
> https://issues.apache.org/jira/browse/LUCY-216
>
> On this ticket you should see 4 attachments:
>
> Inversion.sO0   19kb (Without optimization)
> Inversion.sO0   4kb (Ignore, this was a mistake unable to figure out how to
> delete from Jira)
> Inversion.sO1   16kb (With optimization)

Thanks, Logan. This is definitely a compiler bug. Here's the relevant 
part of the assembler output for the inner loop of Inversion_invert 
(comments are mine):

LBB12_4:
	movq	(%rbx), %rax        # cur_token = *tokens;
	movl	%r9d, 48(%rax)      # cur_token->pos = token_pos;
	movl	44(%rax), %eax      # %eax = cur_token->pos_inc
	testl	%eax, %eax
	leal	(%rax,%r9), %r15d   # %r15d = %eax + token_pos
	jns	LBB12_6
	[ code for THROW(...) ]
LBB12_6:
	addq	$8, %rbx
	cmpq	%r14, %rbx
	movl	%r15d, %r9d
	jb	LBB12_4

The "if" statement in the loop is compiled to the following instructions:

	testl	%eax, %eax
	jns	LBB12_6

Where %eax contains cur_token->pos_inc. So the "if" statement basically 
becomes:

	if (cur_token->pos_inc < 0)

Which is of course bogus. It really makes me wonder why the compiler 
gets this very simple loop wrong. My best guess is that clang tries to 
optimize the standard way to test for integer overflow in C and somehow 
gets it wrong. Since overflows rarely happen, this could explain why 
this bug wasn't noticed early on.

Nick

Mime
View raw message