[Mesa-dev] Mesa (master): r600g: Use default mul/mad function for tgsi-to-llvm

Tom Stellard tom at stellard.net
Thu Dec 6 09:05:53 PST 2012

On Thu, Dec 06, 2012 at 05:08:07PM +0100, Michel Dänzer wrote:
> On Mit, 2012-12-05 at 09:32 -0800, Vincent Lejeune wrote: 
> > Module: Mesa
> > Branch: master
> > Commit: 0ad1fefd6951aa47ab58a41dc9ee73083cbcf85c
> > URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=0ad1fefd6951aa47ab58a41dc9ee73083cbcf85c
> > 
> > Author: Vincent Lejeune <vljn at ovi.com>
> > Date:   Wed Nov 28 00:35:55 2012 +0100
> > 
> > r600g: Use default mul/mad function for tgsi-to-llvm
> This change breaks the piglit tests glsl-{f,v}s-vec4-indexing-temp-src
> on radeonsi. I suspect the same would be true with r600g as well if it
> didn't fall back to the non-LLVM backend for this test.
> Comparing the generated code, I've noticed two main differences so far:
>       * LLVM now optimizes away some TGSI MUL operations with constant
>         1.0, which previously resulted in V_MUL_LEGACY_F32 (non-IEEE
>         semantics) instructions. 
>       * V_MUL_F32 and V_ADD_F32 (IEEE semantics) are used instead of
>         V_MAD_LEGACY_F32 (non-IEEE semantics) in some places.

We really need to fix the SI AsmPrinter and start using the FileCheck tests
in LLVM for things like this.

> I suspect the problem is a (non-)IEEE semantics mismatch between TGSI
> and LLVM introduced by this change.

What are the semantics of TGSI opcodes?  For MUL and MAD, tgsi_exec uses IEEE
operations, but it seems like the glsl frontend thinks they are non-IEEE. 
> BTW, some general issues with this commit:
> The prefix 'r600g:' is misleading, as this change affects radeonsi as
> well. I think we've usually used 'radeon/llvm:' for changes
> src/gallium/drivers/radeon.
> There is no rationale for this change in the commit log: What is the
> intended effect? Why is it a good idea? ...

I agree with you here.  These are things I should have caught during the review.
Should we revert this and the associated LLVM changes?


> -- 
> Earthling Michel Dänzer           |                   http://www.amd.com
> Libre software enthusiast         |          Debian, X and DRI developer
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