Mesa (master): nir/builder: Don't emit no-op swizzles

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Module: Mesa
Branch: master
Commit: 743700be1f583db28ff84104754f38bc6852668e
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=743700be1f583db28ff84104754f38bc6852668e

Author: Jason Ekstrand <jason.ekstrand at intel.com>
Date:   Fri Feb 22 17:06:39 2019 -0600

nir/builder: Don't emit no-op swizzles

The nir_swizzle helper is used some on it's own but it's also called by
nir_channel and nir_channels which are used everywhere.  It's pretty
quick to check while we're walking the swizzle anyway whether or not
it's an identity swizzle.  If it is, we now don't bother emitting the
instruction.  Sure, copy-prop will clean it up for us but there's no
sense making more work for the optimizer than we have to.

Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>

---

 src/compiler/nir/nir_builder.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/compiler/nir/nir_builder.h b/src/compiler/nir/nir_builder.h
index c6e80e729a8..253ca5941cb 100644
--- a/src/compiler/nir/nir_builder.h
+++ b/src/compiler/nir/nir_builder.h
@@ -497,8 +497,16 @@ nir_swizzle(nir_builder *build, nir_ssa_def *src, const unsigned *swiz,
    assert(num_components <= NIR_MAX_VEC_COMPONENTS);
    nir_alu_src alu_src = { NIR_SRC_INIT };
    alu_src.src = nir_src_for_ssa(src);
-   for (unsigned i = 0; i < num_components && i < NIR_MAX_VEC_COMPONENTS; i++)
+
+   bool is_identity_swizzle = true;
+   for (unsigned i = 0; i < num_components && i < NIR_MAX_VEC_COMPONENTS; i++) {
+      if (swiz[i] != i)
+         is_identity_swizzle = false;
       alu_src.swizzle[i] = swiz[i];
+   }
+
+   if (num_components == src->num_components && is_identity_swizzle)
+      return src;
 
    return use_fmov ? nir_fmov_alu(build, alu_src, num_components) :
                      nir_imov_alu(build, alu_src, num_components);