[Mesa-dev] [PATCH 10/23] glsl: Optimize certain if-statements to ir_triop_csel

[Ian Romanick]

From: Ian Romanick <ian.d.romanick at intel.com>

Some shaders end up with code that looks something like:

    if (condition)
        result = <constant A>;
    else
        result = <constant B>;

This pass converts those if-statements to

     result = mix(<constant A>, <constant B>, condition);

Quite a few of the helped shaders are vertex shaders, mostly from Warsow
and Tesseract.  Also, the "instructions in affected programs" percentage
is a bit skewed by a single shader that is really, really helped (FS
SIMD16: 906 -> 634 (-30.02%), FS SIMD8: 871 -> 595 (-31.69%)).

Shader-db results:

GM45 (0x2A42):
total instructions in shared programs: 3545804 -> 3545604 (-0.01%)
instructions in affected programs:     95801 -> 95601 (-0.21%)
helped:                                167
HURT:                                  93

Iron Lake (0x0046):
total instructions in shared programs: 4975982 -> 4975777 (-0.00%)
instructions in affected programs:     94739 -> 94534 (-0.22%)
helped:                                171
HURT:                                  95
GAINED:                                2

Sandy Bridge (0x0116):
total instructions in shared programs: 6803487 -> 6803299 (-0.00%)
instructions in affected programs:     244015 -> 243827 (-0.08%)
helped:                                163
HURT:                                  794

Sandy Bridge (0x0116) NIR:
total instructions in shared programs: 6811992 -> 6811661 (-0.00%)
instructions in affected programs:     10946 -> 10615 (-3.02%)
helped:                                61
HURT:                                  10

Ivy Bridge (0x0166):
total instructions in shared programs: 6279862 -> 6279602 (-0.00%)
instructions in affected programs:     209475 -> 209215 (-0.12%)
helped:                                149
HURT:                                  733

Ivy Bridge (0x0166) NIR:
total instructions in shared programs: 6319460 -> 6319127 (-0.01%)
instructions in affected programs:     10377 -> 10044 (-3.21%)
helped:                                63
HURT:                                  10

Haswell (0x0426):
total instructions in shared programs: 5764623 -> 5764382 (-0.00%)
instructions in affected programs:     188540 -> 188299 (-0.13%)
helped:                                149
HURT:                                  737

Haswell (0x0426) NIR:
total instructions in shared programs: 5794483 -> 5794178 (-0.01%)
instructions in affected programs:     10296 -> 9991 (-2.96%)
helped:                                58
HURT:                                  14

Broadwell (0x162E):
total instructions in shared programs: 6811377 -> 6811611 (0.00%)
instructions in affected programs:     195190 -> 195424 (0.12%)
helped:                                91
HURT:                                  806
GAINED:                                2

Broadwell (0x162E) NIR:
total instructions in shared programs: 7008297 -> 7008390 (0.00%)
instructions in affected programs:     8508 -> 8601 (1.09%)
helped:                                2
HURT:                                  30
LOST:                                  12

Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
---
 src/glsl/opt_if_to_bool_cast.cpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/glsl/opt_if_to_bool_cast.cpp b/src/glsl/opt_if_to_bool_cast.cpp
index 20a27e3..aba9f9e 100644
--- a/src/glsl/opt_if_to_bool_cast.cpp
+++ b/src/glsl/opt_if_to_bool_cast.cpp
@@ -172,6 +172,12 @@ ir_if_to_bool_cast_visitor::visit_leave(ir_if *ir)
       a = emit_0_or_1_assigment(then_assign->lhs, then_assign->write_mask,
                                 ir->condition,
                                 then_rhs, else_rhs);
+   } else {
+      const unsigned size = _mesa_bitcount(then_assign->write_mask);
+
+      a = assign(then_assign->lhs,
+                 csel(swizzle(ir->condition, SWIZZLE_XXXX, size),
+                      then_rhs, else_rhs));
    }
 
    if (a != NULL) {
-- 
2.1.0