[FFmpeg-devel] fate : clang x86

[Måns Rullgård]

Alexander Strange <astrange at ithinksw.com> writes:

> On Aug 23, 2010, at 6:57 PM, M?ns Rullg?rd wrote:
>
>> Eli Friedman <eli.friedman at gmail.com> writes:
>> 
>>> 2010/8/21 M?ns Rullg?rd <mans at mansr.com>:
>>>> Eli Friedman <eli.friedman at gmail.com> writes:
>>>> 
>>>>> 2010/8/21 M?ns Rullg?rd <mans at mansr.com>:
>>>>>> castet.matthieu at free.fr writes:
>>>>>> 
>>>>>>> Hi,
>>>>>>> 
>>>>>>> on freebsd "-mllvm -regalloc=fast" cflags are used to make clang/llvm accept
>>>>>>> some inline asm.
>>>>>>> 
>>>>>>> May be we should do the same on linux ?
>>>>>> 
>>>>>> I tried and failed to figure out what that flag does.  I assume it
>>>>>> does something with the register allocator, but I'd like to know what.
>>>>> 
>>>>> It's a workaround of sorts for
>>>>> http://llvm.org/bugs/show_bug.cgi?id=4668 .  LLVM essentially has two
>>>>> register allocator implementations: one is the "fast" allocator, which
>>>>> is a local register allocator used for -O0, and the other is the
>>>>> "linear scan" allocator, which is the slower global register allocator
>>>>> used for -O1+.  "-mllvm -regalloc=fast" forces the use of the "fast"
>>>>> allocator, which leads to slower generated code, but isn't affected by
>>>>> the bug in question.
>>>> 
>>>> Sounds like it's not suitable for production use.  Any chance they'll
>>>> fix the bug?
>>> 
>>> In the near future?  Not very likely... from what I understand, it's a
>>> relatively difficult issue to solve, and bugs rejecting valid inline
>>> asm are generally considered low priority for the people who know the
>>> register allocator well enough to fix this.
>> 
>> That leaves two options:
>> 
>> 1. Declare clang officially unsupported for x86_32.
>> 2. Fix our code.
>
> For reference, this is the error:
>
> fatal error: error in backend: Ran out of registers during register allocation!
> Please check your inline asm statement for invalid constraints:
> INLINEASM <es:movd %eax, %xmm3              
> 	pshuflw $$0, %xmm3, %xmm3       
> 	punpcklwd %xmm3, %xmm3         
> 	pxor %xmm7, %xmm7                  
> 	pxor %xmm4, %xmm4                  
> 	movdqa ($2), %xmm5                   
> 	pxor %xmm6, %xmm6                  
> 	psubw ($3), %xmm6                  
> 	mov $$-128, %eax               
> 	.p2align 4
> 	1:                                 
> 	movdqa ($1, %eax), %xmm0        
> 	movdqa %xmm0, %xmm1            
> 	pabsw  %xmm0, %xmm0            
> 	psubusw %xmm6, %xmm0               
> 	pmulhw %xmm5, %xmm0                
> 	por %xmm0, %xmm4                   
> 	psignw %xmm1, %xmm0            
> 	movdqa %xmm0, ($5, %eax)        
> 	pcmpeqw %xmm7, %xmm0               
> 	movdqa ($4, %eax), %xmm1        
> 	movdqa %xmm7, ($1, %eax)        
> 	pandn %xmm1, %xmm0                 
> 	pmaxsw %xmm0, %xmm3     
> 	add $$16, %eax      
> 	 js 1b                             
> 	movhlps %xmm3, %xmm0           
> 	pmaxsw %xmm0, %xmm3     
> 	pshuflw $$0x0E, %xmm3, %xmm0    
> 	pmaxsw %xmm0, %xmm3     
> 	pshuflw $$0x01, %xmm3, %xmm0    
> 	pmaxsw %xmm0, %xmm3     
> 	movd %xmm3, %eax              
> 	movzb %al, %eax              
> 	>, 0, 10, %EAX<imp-def>, 9, %reg1309<kill>, 9, %reg1317, 9, %reg1305<kill>, 9, %reg1312<kill>, 9, %reg1310<kill>, 2147483657, %EAX, 14, %EFLAGS<earlyclobber,imp-def,dead>, <!-1>; GR32:%reg1309,1317,1305,1312,1310 dbg:libavcodec/x86/mpegvideo_mmx_template.c:152:9
>
> It only happens (on Darwin x86-32 at least) without
> -fomit-frame-pointer, which is already not the default
> configuration. How are they building it?

I'm using default configure apart from --cc=clang --extra-cflags=-m32.

-- 
M?ns Rullg?rd
mans at mansr.com