[LLVMdev] Auto-vectorization in GCC 4.0

[Chris Lattner]

On Mon, 31 Jul 2006, Devang Patel wrote:
> On Jul 31, 2006, at 11:14 AM, Vikram Adve wrote:
>>  Does llvmgcc4 convert the high-level AST to LLVM (like llvmgcc3x) or does 
>>  it go from GIMPL to LLVM?  If the latter, would it be possible to allow 
>>  some TreeSSA optimizations before emitting LLVM?
> llvmgcc4 intercepts high-level GCC trees to GIMPLE tree transformation 
> routines to get trees that are suitable for LLVM byte code. And TreeSSA 
> optimizer may not be able to handle LLVM suitable GIMPLE trees. One such case 
> is how Array references are handled (&a[i] vs a+i*size). However, I do not 
> know exact details of LLVM byte code generation work inside llvmgcc4.

Another issue is that gimple has various different forms (high gimple, low 
gimple, and several other minor forms).  We work on "high gimple", so 
optimizations that require low gimple or later forms won't work.  I don't 
know what the gcc vectorizor uses, but IIRC it runs late in the pipeline, 
so it probably is low-gimple.

I don't think the gimple->llvm translator can't handle low gimple, but it 
may be possible to extend it.

-Chris

>>  On Jul 31, 2006, at 1:10 PM, Devang Patel wrote:
>> 
>> >  llvmgcc4 emits LLVM byte code before executing GCC optimizations, so one 
>> >  can say that llvmgcc4 disables all GCC optimizations.
>> > 
>> >  On Jul 31, 2006, at 11:01 AM, Zhiru Zhang wrote:
>> > 
>> > >  Hi,
>> > >  I am trying to turn on the new GCC auto-vectorization feature within 
>> > >  llvmgcc4. Below is the command I used, but nothing was vectorized. 
>> > >  Does it mean that llvmgcc4 has disabled this optimization and all I 
>> > >  can do is to embed SSE intrinsics in the source code by hand?
>> > > 
>> > >  Thanks!
>> > > 
>> > >  ./llvm-gcc4-x86/bin/llvm-gcc -c -O2 -ftree-vectorize -msse -ftree- 
>> > >  vectorizer-verbose=5 -emit-llvm vec.c -o vec.bc
>> > > 
>> > >  ////////////////////vec.c/////////////////////////
>> > >  int a[256], b[256], c[256];
>> > >  foo () {
>> > >    int i;
>> > >    for (i=0; i<256; i++){
>> > >     a[i] = b[i] + c[i];
>> > >   }
>> > >  }
>> > > 
>> > > 
>> > >  _______________________________________________
>> > >  LLVM Developers mailing list
>> > >  LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > >  http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> > 
>> >  -
>> >  Devang
>> > 
>> > 
>> > 
>> >  _______________________________________________
>> >  LLVM Developers mailing list
>> >  LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >  http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> 
>>  _______________________________________________
>>  LLVM Developers mailing list
>>  LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>  http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
> -
> Devang
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/