Bug 29199

Summary: Using negation as an argument to a shader function sometimes negates twice
Product: Mesa Reporter: Neil Roberts <nroberts>
Component: glsl-compilerAssignee: Ian Romanick <idr>
Status: RESOLVED WORKSFORME QA Contact:
Severity: normal    
Priority: medium    
Version: git   
Hardware: All   
OS: All   
Whiteboard:
i915 platform: i915 features:
Attachments: Add a shader which does a negation as one of the arguments to dot

Description Neil Roberts 2010-07-21 11:40:04 UTC 
In some situations (I'm not exactly sure what) if you use the unary negate operator in one of the arguments to a function then Mesa generates opcodes which do the negation twice (once in a MOV and once as part of a swizzle).

For example, something like this:

vec3 vec_a;
vec3 vec_b;

/* .... */

float result = dot(-vec_a, vec_b);

generates the following instructions:

MOV TEMP[1].xyz, TEMP[1].-x-y-z-w;           <-- negation once with MOV
MOV (-1 -0 -0 -0)
DP3 TEMP[0].w, TEMP[1].-x-y-z-w, UNIFORM[1]; <-- negation again with swizzle
DP3 -1 = (1 0 0) . (-1 0 0)

Attached is a test for piglit.
Comment 1 Neil Roberts 2010-07-21 11:41:33 UTC 
Created attachment 37276 [details] [review]
Add a shader which does a negation as one of the arguments to dot

This is basically doing:

 vec3 a_vec;
 vec3 b_vec;
 /* some random calculations */
 float result = dot(-a_vec, b_vec);

But for some reason it currently gets compiled as if it was:

 vec3 a_vec;
 vec3 b_vec;
 /* some random calculations */
 float result = dot(--a_vec, b_vec);

Ie, the negation gets done twice.
Comment 2 Ian Romanick 2010-12-14 17:26:29 UTC 
I added this test case to piglit, but it seems to pass with both i965 and swrast.  It seems likely that this was fixed somewhere over the last 5 months, but who knows which commit did it.

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