<div dir="ltr">R-B </div><div class="gmail_extra"> <div class="gmail_quote">On Wed, Jun 1, 2016 at 5:17 PM, Francisco Jerez <<a href="mailto:currojerez@riseup.net" target="_blank">currojerez@riseup.net</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">The conditional mod of these instructions determines the semantics of the comparison itself (rather than being evaluated based on the result of the instruction as is usually the case for most other instructions that allow conditional mods), so it's in general not legal to propagate a conditional mod into a CMP instruction. This prevents cmod propagation from (mis)optimizing: cmp.z.f0 tmp, ... mov.z.f0 null, tmp into: cmp.z.f0 tmp, ... which gives the negation of the flag result of the original sequence. I originally noticed this while working on SIMD32 in the scalar back-end, but the same scenario is likely to be possible in vec4 programs so this commit ports the bugfix with the same name from the scalar back-end to the vec4 cmod propagation pass. Cc: <a href="mailto:mesa-stable@lists.freedesktop.org">mesa-stable@lists.freedesktop.org</a> --- src/mesa/drivers/dri/i965/brw_vec4_cmod_propagation.cpp | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/src/mesa/drivers/dri/i965/brw_vec4_cmod_propagation.cpp b/src/mesa/drivers/dri/i965/brw_vec4_cmod_propagation.cpp index 0c8224f..c376beb 100644 --- a/src/mesa/drivers/dri/i965/brw_vec4_cmod_propagation.cpp +++ b/src/mesa/drivers/dri/i965/brw_vec4_cmod_propagation.cpp @@ -115,6 +115,18 @@ opt_cmod_propagation_local(bblock_t *block) break; } + /* The conditional mod of the CMP/CMPN instructions behaves + * specially because the flag output is not calculated from the + * result of the instruction, but the other way around, which + * means that even if the condmod to propagate and the condmod + * from the CMP instruction are the same they will in general give + * different results because they are evaluated based on different + * inputs. + */ + if (scan_inst->opcode == BRW_OPCODE_CMP || + scan_inst->opcode == BRW_OPCODE_CMPN) + break; + /* Otherwise, try propagating the conditional. */ enum brw_conditional_mod cond = inst->src[0].negate ? brw_swap_cmod(inst->conditional_mod) -- 2.7.3 </blockquote></div> </div>