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Argyrios Kyrtzidis

2010-Aug-31 18:18 UTC

[LLVMdev] "equivalent" .ll files diverge after optimizations are applied

Hi,

I've attached 2 .ll files which are supposed to be equivalent but
'unopt-fail.ll' causes a crash in webkit's test suite while
'unopt-pass.ll' does not. I can't give more details about the crash,
when I run the crashing test it in isolation it passes, when I run the full
suite it crashes; it boggles the mind.

Below I provide the optimized asm that is produced from each file. Could you
give a hint on what is the problem ?
I also attached 't.cpp' which approximates the source that the .ll files
came from.

-Argiris


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$ opt -std-compile-opts unopt-pass.ll -o - | llc -o -

	.section	__TEXT,__text,regular,pure_instructions
	.globl	__ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
	.align	4, 0x90
__ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE: ##
@_ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
## BB#0:
	subq	$24, %rsp
	movq	%rsi, %rdx
	movl	$0, 16(%rsp)
	movl	$0, 20(%rsp)
	movl	$0, 8(%rsp)
	movl	$0, 12(%rsp)
	movq	8(%rdi), %rsi
	leaq	16(%rsp), %rcx
	leaq	8(%rsp), %r8
	callq
__ZN7WebCore5mouniEPNS_15GraphicsContextEPNS_30GraphicsContextPlatformPrivateERKNS_9FloatRectERNS_10FloatPointES8_
	movss	8(%rsp), %xmm1
	movss	12(%rsp), %xmm0
	subss	20(%rsp), %xmm0
	subss	16(%rsp), %xmm1
                                        ## kill: XMM1<def>
XMM1<kill> XMM1<def>
	insertps	$16, %xmm0, %xmm1 ## xmm1 = xmm1[0],xmm0[0],xmm1[2,3]
	movq	16(%rsp), %xmm0
	addq	$24, %rsp
	ret


$ opt -std-compile-opts unopt-fail.ll -o - | llc -o -

	.section	__TEXT,__text,regular,pure_instructions
	.globl	__ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
	.align	4, 0x90
__ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE: ##
@_ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
## BB#0:
	subq	$24, %rsp
	movq	%rsi, %rdx
	movl	$0, 16(%rsp)
	movl	$0, 20(%rsp)
	movl	$0, 8(%rsp)
	movl	$0, 12(%rsp)
	movq	8(%rdi), %rsi
	leaq	16(%rsp), %rcx
	leaq	8(%rsp), %r8
	callq
__ZN7WebCore5mouniEPNS_15GraphicsContextEPNS_30GraphicsContextPlatformPrivateERKNS_9FloatRectERNS_10FloatPointES8_
	movss	8(%rsp), %xmm0
	movss	12(%rsp), %xmm1
	subss	20(%rsp), %xmm1
	subss	16(%rsp), %xmm0
	insertps	$16, %xmm1, %xmm0 ## xmm0 = xmm0[0],xmm1[0],xmm0[2,3]
	movd	%xmm0, %rax
	movd	%rax, %mm0
	movq2dq	%mm0, %xmm1
	movq2dq	%mm0, %xmm2
	punpcklqdq	%xmm2, %xmm1    ## xmm1 = xmm1[0],xmm2[0]
	movq	16(%rsp), %rax
	movd	%rax, %mm0
	movq2dq	%mm0, %xmm0
	punpcklqdq	%xmm2, %xmm0    ## xmm0 = xmm0[0],xmm2[0]
	addq	$24, %rsp
	ret

Dale Johannesen

2010-Aug-31 18:51 UTC

head link

[LLVMdev] "equivalent" .ll files diverge after optimizations are applied

Using MM registers is wrong unless the user has specifically asked for  
it, which doesn't seem to be the case here.
In the awesome MMX architecture, touching an MM register makes  
subsequent x87 operations fail unless an EMMS instruction is issued  
first; none of the compilers here are smart enough to insert EMMS  
instructions in the right places, so the only safe thing is not to use  
these registers.  There is no x87 instruction shown here, but you've  
probably got one in the full test suite and not in the test by itself,  
which fits your data.

Why this is happening is not immediately clear.  It looks like the  
successful code is doing an aggregate copy field-by-field while the  
failing code has lowered this to a memcpy.   I would certainly expect  
the memcpy expansion to be smart enough to avoid using MM registers,  
though; that's a serious bug if it isn't.

	movd	%xmm0, %rax
	movd	%rax, %mm0
	movq2dq	%mm0, %xmm1
	movq2dq	%mm0, %xmm2
	punpcklqdq	%xmm2, %xmm1    ## xmm1 = xmm1[0],xmm2[0]
	movq	16(%rsp), %rax
	movd	%rax, %mm0
	movq2dq	%mm0, %xmm0
	punpcklqdq	%xmm2, %xmm0    ## xmm0 = xmm0[0],xmm2[0]


On Aug 31, 2010, at 11:18 AMPDT, Argyrios Kyrtzidis wrote:
> Hi,
>
> I've attached 2 .ll files which are supposed to be equivalent but  
> 'unopt-fail.ll' causes a crash in webkit's test suite while
'unopt-
> pass.ll' does not. I can't give more details about the crash, when
I
> run the crashing test it in isolation it passes, when I run the full  
> suite it crashes; it boggles the mind.
>
> Below I provide the optimized asm that is produced from each file.  
> Could you give a hint on what is the problem ?
> I also attached 't.cpp' which approximates the source that the .ll
> files came from.
>
> -Argiris
>
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>
>
> $ opt -std-compile-opts unopt-pass.ll -o - | llc -o -
>
> 	.section	__TEXT,__text,regular,pure_instructions
> 	.globl	 
> __ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
> 	.align	4, 0x90
> __ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE:  
> ## @_ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
> ## BB#0:
> 	subq	$24, %rsp
> 	movq	%rsi, %rdx
> 	movl	$0, 16(%rsp)
> 	movl	$0, 20(%rsp)
> 	movl	$0, 8(%rsp)
> 	movl	$0, 12(%rsp)
> 	movq	8(%rdi), %rsi
> 	leaq	16(%rsp), %rcx
> 	leaq	8(%rsp), %r8
> 	callq	 
>
__ZN7WebCore5mouniEPNS_15GraphicsContextEPNS_30GraphicsContextPlatformPrivateERKNS_9FloatRectERNS_10FloatPointES8_
> 	movss	8(%rsp), %xmm1
> 	movss	12(%rsp), %xmm0
> 	subss	20(%rsp), %xmm0
> 	subss	16(%rsp), %xmm1
>                                        ## kill: XMM1<def>
XMM1<kill>
> XMM1<def>
> 	insertps	$16, %xmm0, %xmm1 ## xmm1 = xmm1[0],xmm0[0],xmm1[2,3]
> 	movq	16(%rsp), %xmm0
> 	addq	$24, %rsp
> 	ret
>
>
> $ opt -std-compile-opts unopt-fail.ll -o - | llc -o -
>
> 	.section	__TEXT,__text,regular,pure_instructions
> 	.globl	 
> __ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
> 	.align	4, 0x90
> __ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE:  
> ## @_ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
> ## BB#0:
> 	subq	$24, %rsp
> 	movq	%rsi, %rdx
> 	movl	$0, 16(%rsp)
> 	movl	$0, 20(%rsp)
> 	movl	$0, 8(%rsp)
> 	movl	$0, 12(%rsp)
> 	movq	8(%rdi), %rsi
> 	leaq	16(%rsp), %rcx
> 	leaq	8(%rsp), %r8
> 	callq	 
>
__ZN7WebCore5mouniEPNS_15GraphicsContextEPNS_30GraphicsContextPlatformPrivateERKNS_9FloatRectERNS_10FloatPointES8_
> 	movss	8(%rsp), %xmm0
> 	movss	12(%rsp), %xmm1
> 	subss	20(%rsp), %xmm1
> 	subss	16(%rsp), %xmm0
> 	insertps	$16, %xmm1, %xmm0 ## xmm0 = xmm0[0],xmm1[0],xmm0[2,3]
> 	movd	%xmm0, %rax
> 	movd	%rax, %mm0
> 	movq2dq	%mm0, %xmm1
> 	movq2dq	%mm0, %xmm2
> 	punpcklqdq	%xmm2, %xmm1    ## xmm1 = xmm1[0],xmm2[0]
> 	movq	16(%rsp), %rax
> 	movd	%rax, %mm0
> 	movq2dq	%mm0, %xmm0
> 	punpcklqdq	%xmm2, %xmm0    ## xmm0 = xmm0[0],xmm2[0]
> 	addq	$24, %rsp
> 	ret
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Argyrios Kyrtzidis

2010-Aug-31 20:21 UTC

head link

[LLVMdev] "equivalent" .ll files diverge after optimizations are applied

Here's the optimized versions:

$ opt -std-compile-opts unopt-pass.ll -o - | llvm-dis -o -

[...]
define %3
@_ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE(%"class.WebCore::GraphicsContext"*
%this, %"struct.WebCore::FloatRect"* %rect) nounwind ssp align 2 {
  %roundedOrigin = alloca %"class.WebCore::FloatSize", align 4 ;
<%"class.WebCore::FloatSize"*> [#uses=3]
  %roundedLowerRight = alloca %"class.WebCore::FloatSize", align 4 ;
<%"class.WebCore::FloatSize"*> [#uses=3]
  %1 = getelementptr inbounds %"class.WebCore::FloatSize"*
%roundedOrigin, i64 0, i32 0 ; <float*> [#uses=2]
  store float 0.000000e+00, float* %1, align 4
  %2 = getelementptr inbounds %"class.WebCore::FloatSize"*
%roundedOrigin, i64 0, i32 1 ; <float*> [#uses=2]
  store float 0.000000e+00, float* %2, align 4
  %3 = getelementptr inbounds %"class.WebCore::FloatSize"*
%roundedLowerRight, i64 0, i32 0 ; <float*> [#uses=2]
  store float 0.000000e+00, float* %3, align 4
  %4 = getelementptr inbounds %"class.WebCore::FloatSize"*
%roundedLowerRight, i64 0, i32 1 ; <float*> [#uses=2]
  store float 0.000000e+00, float* %4, align 4
  %5 = getelementptr inbounds %"class.WebCore::GraphicsContext"*
%this, i64 0, i32 1 ;
<%"class.WebCore::GraphicsContextPlatformPrivate"**> [#uses=1]
  %6 = load %"class.WebCore::GraphicsContextPlatformPrivate"** %5,
align 8 ; <%"class.WebCore::GraphicsContextPlatformPrivate"*>
[#uses=1]
  call void
@_ZN7WebCore5mouniEPNS_15GraphicsContextEPNS_30GraphicsContextPlatformPrivateERKNS_9FloatRectERNS_10FloatPointES8_(%"class.WebCore::GraphicsContext"*
%this, %"class.WebCore::GraphicsContextPlatformPrivate"* %6,
%"struct.WebCore::FloatRect"* %rect,
%"class.WebCore::FloatSize"* %roundedOrigin,
%"class.WebCore::FloatSize"* %roundedLowerRight) nounwind
  %7 = load float* %3, align 4                    ; <float> [#uses=1]
  %8 = load float* %1, align 4                    ; <float> [#uses=2]
  %9 = fsub float %7, %8                          ; <float> [#uses=1]
  %10 = load float* %4, align 4                   ; <float> [#uses=1]
  %11 = load float* %2, align 4                   ; <float> [#uses=2]
  %12 = fsub float %10, %11                       ; <float> [#uses=1]
  %13 = insertelement <2 x float> undef, float %8, i32 0 ; <<2 x
float>> [#uses=1]
  %14 = insertelement <2 x float> %13, float %11, i32 1 ; <<2 x
float>> [#uses=1]
  %tmp8 = insertvalue %3 undef, <2 x float> %14, 0 ; <%3> [#uses=1]
  %15 = insertelement <2 x float> undef, float %9, i32 0 ; <<2 x
float>> [#uses=1]
  %16 = insertelement <2 x float> %15, float %12, i32 1 ; <<2 x
float>> [#uses=1]
  %tmp12 = insertvalue %3 %tmp8, <2 x float> %16, 1 ; <%3> [#uses=1]
  ret %3 %tmp12
}

$ opt -std-compile-opts unopt-fail.ll -o - | llvm-dis -o -

[...]
define %3
@_ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE(%"class.WebCore::GraphicsContext"*
%this, %"struct.WebCore::FloatRect"* %rect) nounwind ssp align 2 {
  %roundedOrigin = alloca i64, align 8            ; <i64*> [#uses=3]
  %tmpcast = bitcast i64* %roundedOrigin to
%"class.WebCore::FloatSize"* ;
<%"class.WebCore::FloatSize"*> [#uses=2]
  %roundedLowerRight = alloca %"class.WebCore::FloatSize", align 4 ;
<%"class.WebCore::FloatSize"*> [#uses=3]
  %1 = bitcast i64* %roundedOrigin to float*      ; <float*> [#uses=2]
  store float 0.000000e+00, float* %1, align 8
  %2 = getelementptr inbounds %"class.WebCore::FloatSize"* %tmpcast,
i64 0, i32 1 ; <float*> [#uses=2]
  store float 0.000000e+00, float* %2, align 4
  %3 = getelementptr inbounds %"class.WebCore::FloatSize"*
%roundedLowerRight, i64 0, i32 0 ; <float*> [#uses=2]
  store float 0.000000e+00, float* %3, align 4
  %4 = getelementptr inbounds %"class.WebCore::FloatSize"*
%roundedLowerRight, i64 0, i32 1 ; <float*> [#uses=2]
  store float 0.000000e+00, float* %4, align 4
  %5 = getelementptr inbounds %"class.WebCore::GraphicsContext"*
%this, i64 0, i32 1 ;
<%"class.WebCore::GraphicsContextPlatformPrivate"**> [#uses=1]
  %6 = load %"class.WebCore::GraphicsContextPlatformPrivate"** %5,
align 8 ; <%"class.WebCore::GraphicsContextPlatformPrivate"*>
[#uses=1]
  call void
@_ZN7WebCore5mouniEPNS_15GraphicsContextEPNS_30GraphicsContextPlatformPrivateERKNS_9FloatRectERNS_10FloatPointES8_(%"class.WebCore::GraphicsContext"*
%this, %"class.WebCore::GraphicsContextPlatformPrivate"* %6,
%"struct.WebCore::FloatRect"* %rect,
%"class.WebCore::FloatSize"* %tmpcast,
%"class.WebCore::FloatSize"* %roundedLowerRight) nounwind
  %7 = load float* %3, align 4                    ; <float> [#uses=1]
  %8 = load float* %1, align 8                    ; <float> [#uses=1]
  %9 = fsub float %7, %8                          ; <float> [#uses=1]
  %10 = load float* %4, align 4                   ; <float> [#uses=1]
  %11 = load float* %2, align 4                   ; <float> [#uses=1]
  %12 = fsub float %10, %11                       ; <float> [#uses=1]
  %tmp3.i = insertelement <2 x float> undef, float %9, i32 0 ; <<2 x
float>> [#uses=1]
  %tmp1.i = insertelement <2 x float> %tmp3.i, float %12, i32 1 ;
<<2 x float>> [#uses=1]
  %tmp = bitcast <2 x float> %tmp1.i to i64       ; <i64> [#uses=1]
  %tmp.i.i = load i64* %roundedOrigin, align 8    ; <i64> [#uses=1]
  %tmp9 = insertelement <1 x i64> undef, i64 %tmp.i.i, i32 0 ; <<1 x
i64>> [#uses=1]
  %tmp6 = insertelement <1 x i64> undef, i64 %tmp, i32 0 ; <<1 x
i64>> [#uses=1]
  %tmp11 = bitcast <1 x i64> %tmp9 to <2 x float> ; <<2 x
float>> [#uses=1]
  %insert = insertvalue %3 undef, <2 x float> %tmp11, 0 ; <%3>
[#uses=1]
  %tmp8 = bitcast <1 x i64> %tmp6 to <2 x float>  ; <<2 x
float>> [#uses=1]
  %insert4 = insertvalue %3 %insert, <2 x float> %tmp8, 1 ; <%3>
[#uses=1]
  ret %3 %insert4
}


Just to be clear, are you saying that the fact that, after using llc on the
second IR, the produced asm is using MM registers, indicates a bug ?

-Argiris


On Aug 31, 2010, at 7:51 PM, Dale Johannesen wrote:
> Using MM registers is wrong unless the user has specifically asked for it,
which doesn't seem to be the case here.
> In the awesome MMX architecture, touching an MM register makes subsequent
x87 operations fail unless an EMMS instruction is issued first; none of the
compilers here are smart enough to insert EMMS instructions in the right places,
so the only safe thing is not to use these registers.  There is no x87
instruction shown here, but you've probably got one in the full test suite
and not in the test by itself, which fits your data.
> 
> Why this is happening is not immediately clear.  It looks like the
successful code is doing an aggregate copy field-by-field while the failing code
has lowered this to a memcpy.   I would certainly expect the memcpy expansion to
be smart enough to avoid using MM registers, though; that's a serious bug if
it isn't.
> 
> 	movd	%xmm0, %rax
> 	movd	%rax, %mm0
> 	movq2dq	%mm0, %xmm1
> 	movq2dq	%mm0, %xmm2
> 	punpcklqdq	%xmm2, %xmm1    ## xmm1 = xmm1[0],xmm2[0]
> 	movq	16(%rsp), %rax
> 	movd	%rax, %mm0
> 	movq2dq	%mm0, %xmm0
> 	punpcklqdq	%xmm2, %xmm0    ## xmm0 = xmm0[0],xmm2[0]
> 
> 
> On Aug 31, 2010, at 11:18 AMPDT, Argyrios Kyrtzidis wrote:
> 
>> Hi,
>> 
>> I've attached 2 .ll files which are supposed to be equivalent but
'unopt-fail.ll' causes a crash in webkit's test suite while
'unopt-pass.ll' does not. I can't give more details about the crash,
when I run the crashing test it in isolation it passes, when I run the full
suite it crashes; it boggles the mind.
>> 
>> Below I provide the optimized asm that is produced from each file.
Could you give a hint on what is the problem ?
>> I also attached 't.cpp' which approximates the source that the
.ll files came from.
>> 
>> -Argiris
>> 
>> 
> <unopt-fail.ll><unopt-pass.ll><t.cpp>
>> 
>> 
>> 
>> 
>> $ opt -std-compile-opts unopt-pass.ll -o - | llc -o -
>> 
>> 	.section	__TEXT,__text,regular,pure_instructions
>> 	.globl
__ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
>> 	.align	4, 0x90
>> __ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE: ##
@_ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
>> ## BB#0:
>> 	subq	$24, %rsp
>> 	movq	%rsi, %rdx
>> 	movl	$0, 16(%rsp)
>> 	movl	$0, 20(%rsp)
>> 	movl	$0, 8(%rsp)
>> 	movl	$0, 12(%rsp)
>> 	movq	8(%rdi), %rsi
>> 	leaq	16(%rsp), %rcx
>> 	leaq	8(%rsp), %r8
>> 	callq
__ZN7WebCore5mouniEPNS_15GraphicsContextEPNS_30GraphicsContextPlatformPrivateERKNS_9FloatRectERNS_10FloatPointES8_
>> 	movss	8(%rsp), %xmm1
>> 	movss	12(%rsp), %xmm0
>> 	subss	20(%rsp), %xmm0
>> 	subss	16(%rsp), %xmm1
>>                                       ## kill: XMM1<def>
XMM1<kill> XMM1<def>
>> 	insertps	$16, %xmm0, %xmm1 ## xmm1 = xmm1[0],xmm0[0],xmm1[2,3]
>> 	movq	16(%rsp), %xmm0
>> 	addq	$24, %rsp
>> 	ret
>> 
>> 
>> $ opt -std-compile-opts unopt-fail.ll -o - | llc -o -
>> 
>> 	.section	__TEXT,__text,regular,pure_instructions
>> 	.globl
__ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
>> 	.align	4, 0x90
>> __ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE: ##
@_ZN7WebCore15GraphicsContext19roundToDevicePixelsERKNS_9FloatRectE
>> ## BB#0:
>> 	subq	$24, %rsp
>> 	movq	%rsi, %rdx
>> 	movl	$0, 16(%rsp)
>> 	movl	$0, 20(%rsp)
>> 	movl	$0, 8(%rsp)
>> 	movl	$0, 12(%rsp)
>> 	movq	8(%rdi), %rsi
>> 	leaq	16(%rsp), %rcx
>> 	leaq	8(%rsp), %r8
>> 	callq
__ZN7WebCore5mouniEPNS_15GraphicsContextEPNS_30GraphicsContextPlatformPrivateERKNS_9FloatRectERNS_10FloatPointES8_
>> 	movss	8(%rsp), %xmm0
>> 	movss	12(%rsp), %xmm1
>> 	subss	20(%rsp), %xmm1
>> 	subss	16(%rsp), %xmm0
>> 	insertps	$16, %xmm1, %xmm0 ## xmm0 = xmm0[0],xmm1[0],xmm0[2,3]
>> 	movd	%xmm0, %rax
>> 	movd	%rax, %mm0
>> 	movq2dq	%mm0, %xmm1
>> 	movq2dq	%mm0, %xmm2
>> 	punpcklqdq	%xmm2, %xmm1    ## xmm1 = xmm1[0],xmm2[0]
>> 	movq	16(%rsp), %rax
>> 	movd	%rax, %mm0
>> 	movq2dq	%mm0, %xmm0
>> 	punpcklqdq	%xmm2, %xmm0    ## xmm0 = xmm0[0],xmm2[0]
>> 	addq	$24, %rsp
>> 	ret
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

Chris Lattner

2010-Sep-01 05:19 UTC

head link

[LLVMdev] "equivalent" .ll files diverge after optimizations are applied

On Aug 31, 2010, at 11:18 AM, Argyrios Kyrtzidis wrote:
> Hi,
> 
> I've attached 2 .ll files which are supposed to be equivalent but
'unopt-fail.ll' causes a crash in webkit's test suite while
'unopt-pass.ll' does not. I can't give more details about the crash,
when I run the crashing test it in isolation it passes, when I run the full
suite it crashes; it boggles the mind.
> 
> Below I provide the optimized asm that is produced from each file. Could
you give a hint on what is the problem ?
> I also attached 't.cpp' which approximates the source that the .ll
files came from.
Hi Argiris,

The real problem here is that the X86 backend is turning datatypes like <1 x
i64> into MMX operations, but doesn't do so in a safe way (it's not
inserting the requisite EMMS instructions).  After discussing this with Dale and
Bill, the right fix is to stop mapping generic vectors onto MMX operations. 
This will define away the existing -disable-mmx flag and make stuff like this
impossible.

However, this isn't going to happen in the next couple days, certainly not
in time for the 2.8 release branch on friday.  As such, I checked in a horrible
hack in r112696 that prevents SRoA from introducing mmx specific vector types. 
I'm not aware of a target where those datatypes are actually useful, so this
shouldn't be bad.  On your testcase, no mmx operations are produced.

Please let me know if you see any other MMX stuff being generated.

-Chris

Argyrios Kyrtzidis

2010-Sep-01 09:25 UTC

head link

[LLVMdev] "equivalent" .ll files diverge after optimizations are applied

Thanks for the quick response!

On 1 Sep 2010, at 06:19, Chris Lattner wrote:
> 
> On Aug 31, 2010, at 11:18 AM, Argyrios Kyrtzidis wrote:
> 
>> Hi,
>> 
>> I've attached 2 .ll files which are supposed to be equivalent but
'unopt-fail.ll' causes a crash in webkit's test suite while
'unopt-pass.ll' does not. I can't give more details about the crash,
when I run the crashing test it in isolation it passes, when I run the full
suite it crashes; it boggles the mind.
>> 
>> Below I provide the optimized asm that is produced from each file.
Could you give a hint on what is the problem ?
>> I also attached 't.cpp' which approximates the source that the
.ll files came from.
> 
> Hi Argiris,
> 
> The real problem here is that the X86 backend is turning datatypes like
<1 x i64> into MMX operations, but doesn't do so in a safe way
(it's not inserting the requisite EMMS instructions).  After discussing this
with Dale and Bill, the right fix is to stop mapping generic vectors onto MMX
operations.  This will define away the existing -disable-mmx flag and make stuff
like this impossible.
> 
> However, this isn't going to happen in the next couple days, certainly
not in time for the 2.8 release branch on friday.  As such, I checked in a
horrible hack in r112696 that prevents SRoA from introducing mmx specific vector
types.  I'm not aware of a target where those datatypes are actually useful,
so this shouldn't be bad.  On your testcase, no mmx operations are produced.
> 
> Please let me know if you see any other MMX stuff being generated.
> 
> -Chris

Bob Wilson

2010-Sep-01 16:43 UTC

head link

[LLVMdev] "equivalent" .ll files diverge after optimizations are applied

On Aug 31, 2010, at 10:19 PM, Chris Lattner wrote:> 
> Hi Argiris,
> 
> The real problem here is that the X86 backend is turning datatypes like
<1 x i64> into MMX operations, but doesn't do so in a safe way
(it's not inserting the requisite EMMS instructions).  After discussing this
with Dale and Bill, the right fix is to stop mapping generic vectors onto MMX
operations.  This will define away the existing -disable-mmx flag and make stuff
like this impossible.
> 
> However, this isn't going to happen in the next couple days, certainly
not in time for the 2.8 release branch on friday.  As such, I checked in a
horrible hack in r112696 that prevents SRoA from introducing mmx specific vector
types.  I'm not aware of a target where those datatypes are actually useful,
so this shouldn't be bad.  On your testcase, no mmx operations are produced.
Those types are all used with NEON.  I'll try to do some experiments to
measure the impact of this....

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