Fix undefined behavior in minifloat conversion

Shifting a 32-bit value by 32 or more is undefined behavior in C++. This
previously happened in this code when converting a 32-bit float value to
an 11- or 10-bit minifloat which is too small to be represented as a
denormal, and should produce 0 instead.

Instead of going through this arithmetic for denormals, just test
whether the input value is too small to produce a valid denormal, and
return 0 instead.

Bug: chromium:1117433
Change-Id: I8149996fb6d66d328db45725c4cdb81dc7826a10
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48069
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>

src/System/Half.hpp

diff --git a/src/System/Half.hpp b/src/System/Half.hpp
index 5775141..84025ef 100644
--- a/src/System/Half.hpp
+++ b/src/System/Half.hpp
@@ -174,7 +174,8 @@
 		const unsigned int float11ExponentBias = 14;
 
 		const unsigned int float32Maxfloat11 = 0x477E0000;
-		const unsigned int float32Minfloat11 = 0x38800000;
+		const unsigned int float32MinNormfloat11 = 0x38800000;
+		const unsigned int float32MinDenormfloat11 = 0x35000080;
 
 		const unsigned int float32Bits = *reinterpret_cast<unsigned int *>(&fp32);
 		const bool float32Sign = (float32Bits & float32SignMask) == float32SignMask;
@@ -210,9 +211,14 @@
 			// The number is too large to be represented as a float11, set to max
 			return float11Max;
 		}
+		else if(float32Val < float32MinDenormfloat11)
+		{
+			// The number is too small to be represented as a denormalized float11, set to 0
+			return 0;
+		}
 		else
 		{
-			if(float32Val < float32Minfloat11)
+			if(float32Val < float32MinNormfloat11)
 			{
 				// The number is too small to be represented as a normalized float11
 				// Convert it to a denormalized value.
@@ -247,7 +253,8 @@
 		const unsigned int float10ExponentBias = 14;
 
 		const unsigned int float32Maxfloat10 = 0x477C0000;
-		const unsigned int float32Minfloat10 = 0x38800000;
+		const unsigned int float32MinNormfloat10 = 0x38800000;
+		const unsigned int float32MinDenormfloat10 = 0x35800040;
 
 		const unsigned int float32Bits = *reinterpret_cast<unsigned int *>(&fp32);
 		const bool float32Sign = (float32Bits & float32SignMask) == float32SignMask;
@@ -283,9 +290,14 @@
 			// The number is too large to be represented as a float10, set to max
 			return float10Max;
 		}
+		else if(float32Val < float32MinDenormfloat10)
+		{
+			// The number is too small to be represented as a denormalized float10, set to 0
+			return 0;
+		}
 		else
 		{
-			if(float32Val < float32Minfloat10)
+			if(float32Val < float32MinNormfloat10)
 			{
 				// The number is too small to be represented as a normalized float10
 				// Convert it to a denormalized value.