commit 9685e524d34d997aa0a5b8e6b6ac6893399f59a3
author John Stiles <johnstiles@google.com> Mon Dec 14 11:52:14 2020 -0500
committer Skia Commit-Bot <skia-commit-bot@chromium.org> Mon Dec 14 18:07:13 2020 +0000
tree 175fa0bc572cb6fe19e165bd698e42d72495dea2
parent bdbfcd2374a7c4406f2b8cd83d1762e21f744994

Implement findMSB intrinsic in Metal.

findMSB has one special trick that Metal doesn't naturally have an
equivalent for, specifically in its treatment of negative numbers.
findMSB searches negative numbers for a zero bit, not a one bit!
We emulate this behavior in Metal using select(n, ~n, n<0).

Change-Id: I861c6b8fb3dc5427643cd8c68a39a53f1959bff3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/343996
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>

src/sksl/SkSLMetalCodeGenerator.cpp

diff --git a/src/sksl/SkSLMetalCodeGenerator.cpp b/src/sksl/SkSLMetalCodeGenerator.cpp
index af94a88..8cafe20 100644
--- a/src/sksl/SkSLMetalCodeGenerator.cpp
+++ b/src/sksl/SkSLMetalCodeGenerator.cpp
@@ -50,6 +50,7 @@
     fIntrinsicMap[String("dot")]                = SPECIAL(Dot);
     fIntrinsicMap[String("faceforward")]        = SPECIAL(Faceforward);
     fIntrinsicMap[String("findLSB")]            = SPECIAL(FindLSB);
+    fIntrinsicMap[String("findMSB")]            = SPECIAL(FindMSB);
     fIntrinsicMap[String("length")]             = SPECIAL(Length);
     fIntrinsicMap[String("mod")]                = SPECIAL(Mod);
     fIntrinsicMap[String("normalize")]          = SPECIAL(Normalize);
@@ -700,6 +701,54 @@
             this->write("(0)))");
             break;
         }
+        case kFindMSB_SpecialIntrinsic: {
+            // Create a temp variable to store the expression, to avoid double-evaluating it.
+            String skTemp1 = this->getTempVariable(arguments[0]->type());
+            String exprType = this->typeName(arguments[0]->type());
+
+            // GLSL findMSB is actually quite different from Metal's clz:
+            // - For signed negative numbers, it returns the first zero bit, not the first one bit!
+            // - For an empty input (0/~0 depending on sign), findMSB gives -1; clz is numbits(type)
+
+            // (_skTemp1 = (.....),
+            this->write("(");
+            this->write(skTemp1);
+            this->write(" = (");
+            this->writeExpression(*arguments[0], kSequence_Precedence);
+            this->write("), ");
+
+            // Signed input types might be negative; we need another helper variable to negate the
+            // input (since we can only find one bits, not zero bits).
+            String skTemp2;
+            if (arguments[0]->type().isSigned()) {
+                // ... _skTemp2 = (select(_skTemp1, ~_skTemp1, _skTemp1 < 0)),
+                skTemp2 = this->getTempVariable(arguments[0]->type());
+                this->write(skTemp2);
+                this->write(" = (select(");
+                this->write(skTemp1);
+                this->write(", ~");
+                this->write(skTemp1);
+                this->write(", ");
+                this->write(skTemp1);
+                this->write(" < 0)), ");
+            } else {
+                skTemp2 = skTemp1;
+            }
+
+            // ... select(int4(clz(_skTemp2)), int4(-1), _skTemp2 == int4(0)))
+            this->write("select(");
+            this->write(this->typeName(c.type()));
+            this->write("(clz(");
+            this->write(skTemp2);
+            this->write(")), ");
+            this->write(this->typeName(c.type()));
+            this->write("(-1), ");
+            this->write(skTemp2);
+            this->write(" == ");
+            this->write(exprType);
+            this->write("(0)))");
+            break;
+        }
         default:
             ABORT("unsupported special intrinsic kind");
     }

src/sksl/SkSLMetalCodeGenerator.h

diff --git a/src/sksl/SkSLMetalCodeGenerator.h b/src/sksl/SkSLMetalCodeGenerator.h
index cfe955a..7fbe031 100644
--- a/src/sksl/SkSLMetalCodeGenerator.h
+++ b/src/sksl/SkSLMetalCodeGenerator.h
@@ -110,6 +110,7 @@
         kDot_SpecialIntrinsic,
         kFaceforward_SpecialIntrinsic,
         kFindLSB_SpecialIntrinsic,
+        kFindMSB_SpecialIntrinsic,
         kLength_SpecialIntrinsic,
         kMod_SpecialIntrinsic,
         kNormalize_SpecialIntrinsic,

tests/sksl/intrinsics/golden/FindMSB.metal

diff --git a/tests/sksl/intrinsics/golden/FindMSB.metal b/tests/sksl/intrinsics/golden/FindMSB.metal
index a6d94e0..3a9e1e6 100644
--- a/tests/sksl/intrinsics/golden/FindMSB.metal
+++ b/tests/sksl/intrinsics/golden/FindMSB.metal
@@ -13,7 +13,10 @@
 fragment Outputs fragmentMain(Inputs _in [[stage_in]], bool _frontFacing [[front_facing]], float4 _fragCoord [[position]]) {
     Outputs _outputStruct;
     thread Outputs* _out = &_outputStruct;
-    _out->sk_FragColor.x = float(findMSB(_in.a));
-    _out->sk_FragColor.y = float(findMSB(_in.b));
+    int _skTemp0;
+    int _skTemp1;
+    uint _skTemp2;
+    _out->sk_FragColor.x = float((_skTemp0 = (_in.a), _skTemp1 = (select(_skTemp0, ~_skTemp0, _skTemp0 < 0)), select(int(clz(_skTemp1)), int(-1), _skTemp1 == int(0))));
+    _out->sk_FragColor.y = float((_skTemp2 = (_in.b), select(int(clz(_skTemp2)), int(-1), _skTemp2 == uint(0))));
     return *_out;
 }