SpirvShader: Implement GLSLstd450FindILsb

Bug: b/126873455
Tests: dEQP-VK.glsl.builtin.function.integer.findlsb.*
Change-Id: I46671fe6b64814a5c9cbc8dd9fe4cc449a328f42
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28789
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

@@ -3605,7 +3605,12 @@ namespace sw
 		}
 		case GLSLstd450FindILsb:
 		{
-			UNIMPLEMENTED("GLSLstd450FindILsb");
+			auto val = GenericValue(this, routine, insn.word(5));
+			for (auto i = 0u; i < type.sizeInComponents; i++)
+			{
+				auto v = val.UInt(i);
+				dst.move(i, Cttz(v, true) | CmpEQ(v, SIMD::UInt(0)));
+			}
 			break;
 		}
 		case GLSLstd450FindSMsb:

src/Reactor/LLVMReactor.cpp

@@ -3219,6 +3219,15 @@ namespace rr
 		})));
 	}
 
+	RValue<UInt4> Cttz(RValue<UInt4> v, bool isZeroUndef)
+	{
+		auto func = llvm::Intrinsic::getDeclaration(::module, llvm::Intrinsic::cttz, { 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

@@ -2238,6 +2238,11 @@ namespace rr
 	// Returns an undefined value when: !isZeroUndef && x == 0.
 	RValue<UInt4> Ctlz(RValue<UInt4> x, bool isZeroUndef);
 
+	// Count trailing zeros.
+	// Returns 32 when: isZeroUndef && x == 0.
+	// Returns an undefined value when: !isZeroUndef && x == 0.
+	RValue<UInt4> Cttz(RValue<UInt4> x, bool isZeroUndef);
+
 	template<class T>
 	class Pointer : public LValue<Pointer<T>>
 	{