Comment # 12
on bug 97988
from Tom Stellard
(In reply to Marek Olšák from comment #11) > Thanks Grigori. > > GLSL: > color = fract(gl_FragCoord.y / 2) < 0.5 ? > texture(texture0, texcoord0) : > texture(texture1, texcoord0); > > texture0 and texture1 are SMEM loads. > > LLVM (SimplifyCFG) transforms it to: > color = texture(fract(gl_FragCoord.y / 2) < 0.5 ? texture0 : texture1, > texcoord0); > > That's a nice transformation. You don't have to use 2 SMEM loads, you can > just use one SMEM load depending on the result of the condition. > > The problem is gl_FragCoord.y is a VGPR and texture0/1 are SGPRs, therefore > flat VMEM loads are used to load the descriptors. However, image_sample > requires descriptors in SGPRs, so v_readfirstlane is used. That effectively > uses the result of the condition from the first active lane, discarding the > results from all other lanes. The result would be exactly the same if the > compiler did: v_readfirstlane s0, gl_FragCoord.y; > > The test case seems pretty trivial I wonder how many other apps are affected. I think the best solution here would be to teach the backend how to do a scalar select based on value of vccz.
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