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Benedict Gaster

2008-Jul-18 13:45 UTC

[LLVMdev] Alignment of vectors

Consider the following C code:

typedef __attribute__(( ext_vector_type(2) )) float float2;
typedef __attribute__(( ext_vector_type(2) )) __attribute__(( aligned(4) ))
float float2_align2;

void foo(void)
{
  const float * p;
  size_t offset;
  float2 tmp = *((float2_align2 *)(p+offset));
}

When compiled with clang emit-llvm I get:

define void @foo() {
entry:
    %p = alloca float*, align 4        ; <float**> [#uses=1]
    %offset = alloca i32, align 4        ; <i32*> [#uses=1]
    %tmp = alloca <2 x float>, align 8        ; <<2 x float>*>
[#uses=1]
    %tmp1 = load float** %p        ; <float*> [#uses=1]
    %tmp2 = load i32* %offset        ; <i32> [#uses=1]
    %add.ptr = getelementptr float* %tmp1, i32 %tmp2        ; <float*>
[#uses=1]
    %conv = bitcast float* %add.ptr to <2 x float>*        ; <<2 x
float>*>
[#uses=1]
    %tmp3 = load <2 x float>* %conv        ; <<2 x float>>
[#uses=1]
    store <2 x float> %tmp3, <2 x float>* %tmp
    ret void
}

The problem is that the load into tmp3 seems to have lost any information
that %conv should not be aligned to 8 bytes but rather 4. Of course, GCC
only states that the alignment attribute will try and enforce a minimal
alignment and so the above code generated by clang is valid but what about
if the following code had been generated:

define void @foo() {
entry:
    %p = alloca float*, align 4        ; <float**> [#uses=1]
    %offset = alloca i32, align 4        ; <i32*> [#uses=1]
    %tmp = alloca <2 x float>, align 4        ; <<2 x float>*>
[#uses=1]
    %tmp1 = load float** %p        ; <float*> [#uses=1]
    %tmp2 = load i32* %offset        ; <i32> [#uses=1]
    %add.ptr = getelementptr float* %tmp1, i32 %tmp2        ; <float*>
[#uses=1]
    %conv = bitcast float* %add.ptr to <2 x float>*        ; <<2 x
float>*>
[#uses=1]
    %tmp3 = load <2 x float>* %conv        ; <<2 x float>>
[#uses=1]
    store <2 x float> %tmp3, <2 x float>* %tmp
    ret void
}

I¹m assuming that, in general, this is not correct as the code:

    %tmp3 = load <2 x float>* %conv        ; <<2 x float>>
[#uses=1]

lacks the now necessary information that conv is unaligned and instead must
be written as:

    %tmp3 = load <2 x float>*%conv, align 2         ; <<2 x
float>>, align2
[#uses=1]

Of course, it is now up to the backend to assure that an unaligned load is
handled correctly in hardware, but assuming this is ok, then is this
correct?

Thanks,

Ben
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Eli Friedman

2008-Jul-18 15:30 UTC

head link

[LLVMdev] Alignment of vectors

On Fri, Jul 18, 2008 at 6:45 AM, Benedict Gaster
<benedict.gaster at amd.com> wrote:> Consider the following C code:
>
> typedef __attribute__(( ext_vector_type(2) )) float float2;
> typedef __attribute__(( ext_vector_type(2) )) __attribute__(( aligned(4) ))
AFAIK, the aligned attribute doesn't do anything on a typedef of
anything other than a struct/union type in either gcc or clang.  It
would be possible to implement something like this, but someone would
need to spec out exactly what it would do in various situations and
send the proposal past the gcc devs.
> float float2_align2;
>
> void foo(void)
> {
>   const float * p;
>   size_t offset;
>   float2 tmp = *((float2_align2 *)(p+offset));
> }
>
> When compiled with clang —emit-llvm I get:
>
> define void @foo() {
> entry:
>     %p = alloca float*, align 4        ; <float**> [#uses=1]
>     %offset = alloca i32, align 4        ; <i32*> [#uses=1]
>     %tmp = alloca <2 x float>, align 8        ; <<2 x
float>*> [#uses=1]
>     %tmp1 = load float** %p        ; <float*> [#uses=1]
>     %tmp2 = load i32* %offset        ; <i32> [#uses=1]
>     %add.ptr = getelementptr float* %tmp1, i32 %tmp2        ;
<float*>
> [#uses=1]
>     %conv = bitcast float* %add.ptr to <2 x float>*        ;
<<2 x float>*>
> [#uses=1]
>     %tmp3 = load <2 x float>* %conv        ; <<2 x
float>> [#uses=1]
>     store <2 x float> %tmp3, <2 x float>* %tmp
>     ret void
> }
>
> The problem is that the load into tmp3 seems to have lost any information
> that %conv should not be aligned to 8 bytes but rather 4. Of course, GCC
> only states that the alignment attribute will try and enforce a minimal
> alignment and so the above code generated by clang is valid
Right.
> but what about
> if the following code had been generated:
>
> define void @foo() {
> entry:
>     %p = alloca float*, align 4        ; <float**> [#uses=1]
>     %offset = alloca i32, align 4        ; <i32*> [#uses=1]
>     %tmp = alloca <2 x float>, align 4        ; <<2 x
float>*> [#uses=1]
>     %tmp1 = load float** %p        ; <float*> [#uses=1]
>     %tmp2 = load i32* %offset        ; <i32> [#uses=1]
>     %add.ptr = getelementptr float* %tmp1, i32 %tmp2        ;
<float*>
> [#uses=1]
>     %conv = bitcast float* %add.ptr to <2 x float>*        ;
<<2 x float>*>
> [#uses=1]
>     %tmp3 = load <2 x float>* %conv        ; <<2 x
float>> [#uses=1]
>     store <2 x float> %tmp3, <2 x float>* %tmp
>     ret void
> }
clang shouldn't be generating code like that.  Per the current gcc
documentation, the aligned attribute isn't allowed to decrease
alignment.  What would happen if the code subsequently passed a
pointer to the variable with the aligned attribute to another
function?
> Of course, it is now up to the backend to assure that an unaligned load is
> handled correctly in hardware, but assuming this is ok, then is this
> correct?
Unaligned loads should codegen correctly in LLVM; if they don't,
please file a bug.

-Eli

Benedict Gaster

2008-Jul-18 17:12 UTC

head link

[LLVMdev] Alignment of vectors

Hi,

Comments inline.

Ben


On 18/07/2008 16:30, "Eli Friedman" <eli.friedman at gmail.com>
wrote:
> On Fri, Jul 18, 2008 at 6:45 AM, Benedict Gaster
> <benedict.gaster at amd.com> wrote:
>> Consider the following C code:
>> 
>> typedef __attribute__(( ext_vector_type(2) )) float float2;
>> typedef __attribute__(( ext_vector_type(2) )) __attribute__((
aligned(4) ))
> 
> AFAIK, the aligned attribute doesn't do anything on a typedef of
> anything other than a struct/union type in either gcc or clang.  It
> would be possible to implement something like this, but someone would
> need to spec out exactly what it would do in various situations and
> send the proposal past the gcc devs.
> [bg] Thanks, I missed that.
>> float float2_align2;
>> 
>> void foo(void)
>> {
>>   const float * p;
>>   size_t offset;
>>   float2 tmp = *((float2_align2 *)(p+offset));
>> }
>> 
>> When compiled with clang emit-llvm I get:
>> 
>> define void @foo() {
>> entry:
>>     %p = alloca float*, align 4        ; <float**> [#uses=1]
>>     %offset = alloca i32, align 4        ; <i32*> [#uses=1]
>>     %tmp = alloca <2 x float>, align 8        ; <<2 x
float>*> [#uses=1]
>>     %tmp1 = load float** %p        ; <float*> [#uses=1]
>>     %tmp2 = load i32* %offset        ; <i32> [#uses=1]
>>     %add.ptr = getelementptr float* %tmp1, i32 %tmp2        ;
<float*>
>> [#uses=1]
>>     %conv = bitcast float* %add.ptr to <2 x float>*        ;
<<2 x float>*>
>> [#uses=1]
>>     %tmp3 = load <2 x float>* %conv        ; <<2 x
float>> [#uses=1]
>>     store <2 x float> %tmp3, <2 x float>* %tmp
>>     ret void
>> }
>> 
>> The problem is that the load into tmp3 seems to have lost any
information
>> that %conv should not be aligned to 8 bytes but rather 4. Of course,
GCC
>> only states that the alignment attribute will try and enforce a minimal
>> alignment and so the above code generated by clang is valid
> 
> Right.
> 
>> but what about
>> if the following code had been generated:
>> 
>> define void @foo() {
>> entry:
>>     %p = alloca float*, align 4        ; <float**> [#uses=1]
>>     %offset = alloca i32, align 4        ; <i32*> [#uses=1]
>>     %tmp = alloca <2 x float>, align 4        ; <<2 x
float>*> [#uses=1]
>>     %tmp1 = load float** %p        ; <float*> [#uses=1]
>>     %tmp2 = load i32* %offset        ; <i32> [#uses=1]
>>     %add.ptr = getelementptr float* %tmp1, i32 %tmp2        ;
<float*>
>> [#uses=1]
>>     %conv = bitcast float* %add.ptr to <2 x float>*        ;
<<2 x float>*>
>> [#uses=1]
>>     %tmp3 = load <2 x float>* %conv        ; <<2 x
float>> [#uses=1]
>>     store <2 x float> %tmp3, <2 x float>* %tmp
>>     ret void
>> }
> 
> clang shouldn't be generating code like that.  Per the current gcc
> documentation, the aligned attribute isn't allowed to decrease
> alignment.  What would happen if the code subsequently passed a
> pointer to the variable with the aligned attribute to another
> function?
> 
[bg] Sorry I did not mean to imply that Clang generated this code, it was
written by hand. What I was trying to understand was: is if it is valid LLVM
IL? I agree that any frontend generating this code must carefully enforce
that restricted alignment pointers are not passed to functions that don't
expect them.
>> Of course, it is now up to the backend to assure that an unaligned load
is
>> handled correctly in hardware, but assuming this is ok, then is this
>> correct?
> 
> Unaligned loads should codegen correctly in LLVM; if they don't,
> please file a bug.
>
> -Eli
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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