R-B On Wed, Jun 1, 2016 at 5:17 PM, Francisco Jerez <curroje...@riseup.net> wrote:
> 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: mesa-sta...@lists.freedesktop.org > --- > 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 > >
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