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@intel.com>

diff --git a/src/compiler/nir/nir_builder.h b/src/compiler/nir/nir_builder.h
index c6e80e729a85d2725831b1fe066a03e742f8c829..253ca5941cb1ded063907232d8497442823566e8 100644 (file)
--- 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);