From ae5984082b5e7aa4e7c4c8f9201f83bca067f66e Mon Sep 17 00:00:00 2001
From: John Stiles <johnstiles@google.com>
Date: Thu, 24 Mar 2022 16:35:32 -0400
Subject: [PATCH] Optimize SkVM selects.

The expression `!x ? y : z` can be optimized to `x ? z : y`, saving a
bit-not. SkVM now supports this optimization.

Change-Id: I06a0d2a716947de1021ba66b054b92e25568c641
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/524226
Reviewed-by: Arman Uguray <armansito@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
---
 src/core/SkVM.cpp                             | 12 ++++---
 .../sksl/runtime/ConversionConstructors.skvm  | 36 +++++++++----------
 2 files changed, 25 insertions(+), 23 deletions(-)

diff --git a/src/core/SkVM.cpp b/src/core/SkVM.cpp
index 7c3e86f3b6..888205b0a2 100644
--- a/src/core/SkVM.cpp
+++ b/src/core/SkVM.cpp
@@ -1163,10 +1163,14 @@ namespace skvm {
     I32 Builder::select(I32 x, I32 y, I32 z) {
         if (y.id == z.id) { return y; }
         if (int X,Y,Z; this->allImm(x.id,&X, y.id,&Y, z.id,&Z)) { return splat(X?Y:Z); }
-        if (this->isImm(x.id,~0)) { return y; }               // true  ? y : z == y
-        if (this->isImm(x.id, 0)) { return z; }               // false ? y : z == z
-        if (this->isImm(y.id, 0)) { return bit_clear(z,x); }  //     x ? 0 : z == ~x&z
-        if (this->isImm(z.id, 0)) { return bit_and  (y,x); }  //     x ? y : 0 ==  x&y
+        if (this->isImm(x.id,~0)) { return y; }                // (true  ? y : z) == y
+        if (this->isImm(x.id, 0)) { return z; }                // (false ? y : z) == z
+        if (this->isImm(y.id, 0)) { return bit_clear(z,x); }   //     (x ? 0 : z) == ~x&z
+        if (this->isImm(z.id, 0)) { return bit_and  (y,x); }   //     (x ? y : 0) ==  x&y
+        if (Val notX = this->holdsBitNot(x.id); notX != NA) {  //    (!x ? y : z) == (x ? z : y)
+            x.id = notX;
+            std::swap(y, z);
+        }
         return {this, this->push(Op::select, x.id, y.id, z.id)};
     }
 
diff --git a/tests/sksl/runtime/ConversionConstructors.skvm b/tests/sksl/runtime/ConversionConstructors.skvm
index 3d61ae44f9..fa477085e7 100644
--- a/tests/sksl/runtime/ConversionConstructors.skvm
+++ b/tests/sksl/runtime/ConversionConstructors.skvm
@@ -1,24 +1,22 @@
-8 registers, 22 instructions:
+7 registers, 20 instructions:
 0	r0 = uniform32 ptr0 4
 1	r1 = uniform32 ptr0 8
 2	r2 = uniform32 ptr0 C
 3	r3 = splat 0 (0)
-4	r4 = splat FFFFFFFF (nan)
-5	r5 = neq_f32 r0 r3
-6	r6 = splat 3F800000 (1)
-7	r6 = bit_and r2 r6
-8	r7 = to_f32 r1
-9	r7 = select r5 r6 r7
-10	r3 = eq_i32 r1 r3
-11	r4 = bit_xor r3 r4
-12	r3 = splat 1 (1.4012985e-45)
-13	r3 = bit_and r2 r3
-14	r0 = trunc r0
-15	r0 = select r4 r3 r0
-16	r0 = to_f32 r0
-17	r0 = mul_f32 r7 r0
+4	r4 = neq_f32 r0 r3
+5	r5 = splat 3F800000 (1)
+6	r5 = bit_and r2 r5
+7	r6 = to_f32 r1
+8	r6 = select r4 r5 r6
+9	r3 = eq_i32 r1 r3
+10	r1 = splat 1 (1.4012985e-45)
+11	r1 = bit_and r2 r1
+12	r0 = trunc r0
+13	r1 = select r3 r0 r1
+14	r1 = to_f32 r1
+15	r1 = mul_f32 r6 r1
 loop:
-18	    store32 ptr1 r0
-19	    store32 ptr2 r0
-20	    store32 ptr3 r0
-21	    store32 ptr4 r0
+16	    store32 ptr1 r1
+17	    store32 ptr2 r1
+18	    store32 ptr3 r1
+19	    store32 ptr4 r1