SpirvShader: Implement GLSLstd450Find[U,S]Msb

Bug: b/126873455
Tests: dEQP-VK.glsl.builtin.function.integer.findMSB.*
Change-Id: I6337998f725334c31d8adeae05fc441bb3895908
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28788
Tested-by: Ben Clayton <bclayton@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>

src/Pipeline/SpirvShader.cpp

diff --git a/src/Pipeline/SpirvShader.cpp b/src/Pipeline/SpirvShader.cpp
index e79fa94..8dc93c6 100644
--- a/src/Pipeline/SpirvShader.cpp
+++ b/src/Pipeline/SpirvShader.cpp
@@ -3610,12 +3610,21 @@
 		}
 		case GLSLstd450FindSMsb:
 		{
-			UNIMPLEMENTED("GLSLstd450FindSMsb");
+			auto val = GenericValue(this, routine, insn.word(5));
+			for (auto i = 0u; i < type.sizeInComponents; i++)
+			{
+				auto v = val.UInt(i) ^ As<SIMD::UInt>(CmpLT(val.Int(i), SIMD::Int(0)));
+				dst.move(i, SIMD::UInt(31) - Ctlz(v, false));
+			}
 			break;
 		}
 		case GLSLstd450FindUMsb:
 		{
-			UNIMPLEMENTED("GLSLstd450FindUMsb");
+			auto val = GenericValue(this, routine, insn.word(5));
+			for (auto i = 0u; i < type.sizeInComponents; i++)
+			{
+				dst.move(i, SIMD::UInt(31) - Ctlz(val.UInt(i), false));
+			}
 			break;
 		}
 		case GLSLstd450InterpolateAtCentroid:

src/Reactor/LLVMReactor.cpp

diff --git a/src/Reactor/LLVMReactor.cpp b/src/Reactor/LLVMReactor.cpp
index 8b3be66..2508508 100644
--- a/src/Reactor/LLVMReactor.cpp
+++ b/src/Reactor/LLVMReactor.cpp
@@ -3210,6 +3210,15 @@
 		return RValue<Float4>(V(::builder->CreateCall(func, { V(v.value) })));
 	}
 
+	RValue<UInt4> Ctlz(RValue<UInt4> v, bool isZeroUndef)
+	{
+		auto func = llvm::Intrinsic::getDeclaration(::module, llvm::Intrinsic::ctlz, { T(UInt4::getType()), T(Bool::getType()) } );
+		return RValue<UInt4>(V(::builder->CreateCall(func, {
+			V(v.value),
+			isZeroUndef ? ::llvm::ConstantInt::getTrue(*::context) : ::llvm::ConstantInt::getFalse(*::context)
+		})));
+	}
+
 	Type *Float4::getType()
 	{
 		return T(llvm::VectorType::get(T(Float::getType()), 4));

src/Reactor/Reactor.hpp

diff --git a/src/Reactor/Reactor.hpp b/src/Reactor/Reactor.hpp
index 4e74812..69a73ba 100644
--- a/src/Reactor/Reactor.hpp
+++ b/src/Reactor/Reactor.hpp
@@ -2230,6 +2230,14 @@
 	RValue<Float4> Exp2(RValue<Float4> x);
 	RValue<Float4> Log2(RValue<Float4> x);
 
+	// Bit Manipulation functions.
+	// TODO: Currentlhy unimplemented for Subzero.
+
+	// Count leading zeros.
+	// Returns 32 when: isZeroUndef && x == 0.
+	// Returns an undefined value when: !isZeroUndef && x == 0.
+	RValue<UInt4> Ctlz(RValue<UInt4> x, bool isZeroUndef);
+
 	template<class T>
 	class Pointer : public LValue<Pointer<T>>
 	{