[LLVMdev] equivalent IR, different asm

[Chris Lattner]

On Sep 1, 2010, at 11:14 AM, Dale Johannesen wrote:

> 
> On Sep 1, 2010, at 6:25 AMPDT, Argyrios Kyrtzidis wrote:
> 
>> The attached .ll files seem equivalent, but the resulting asm from  
>> 'opt-fail.ll' causes a crash to webkit.
>> I suspect the usage of registers is wrong, can someone take a look ?
> 
> Yes, the code here is wrong:
> 
>> 	movl	(%rbx), %ecx
>> 	imull	%ecx, %eax
> 
> This computes h*((int32)%1) in %eax.
> 
>> 	shrq	$32, %rax
>> 	movl	%eax, %ecx
> 
> This is trying to compute (int32)(%1>>32) into %ecx, but is using the  
> wrong input value since %rax has been clobbered by the above code, and  
> further is clobbering the value in %eax computed above, which is  
> implicit input to the divide.  This is some kind of back end error,  
> probably register allocator.

Jakob, can you take a look when you get a chance?

-Chris