Fix Frac() returning 1.0.
Frac() should always produce results in the range [0.0, 1.0), and thus
never produce a 1.0. However, the current implementation uses
x - floor(x) and this returns 1.0 for very small negative values due to
catastrophic cancellation.
Bug swiftshader:74
Change-Id: I942dd7cfb1f7ee3a260070e748704f005eed0b13
Reviewed-on: https://swiftshader-review.googlesource.com/10648
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
diff --git a/src/Reactor/LLVMReactor.cpp b/src/Reactor/LLVMReactor.cpp
index dda5c04..d8bda70 100644
--- a/src/Reactor/LLVMReactor.cpp
+++ b/src/Reactor/LLVMReactor.cpp
@@ -6255,16 +6255,22 @@
RValue<Float4> Frac(RValue<Float4> x)
{
+ Float4 frc;
+
if(CPUID::supportsSSE4_1())
{
- return x - x86::floorps(x);
+ frc = x - x86::floorps(x);
}
else
{
- Float4 frc = x - Float4(Int4(x)); // Signed fractional part
+ frc = x - Float4(Int4(x)); // Signed fractional part.
- return frc + As<Float4>(As<Int4>(CmpNLE(Float4(0.0f), frc)) & As<Int4>(Float4(1, 1, 1, 1)));
+ frc += As<Float4>(As<Int4>(CmpNLE(Float4(0.0f), frc)) & As<Int4>(Float4(1.0f))); // Add 1.0 if negative.
}
+
+ // x - floor(x) can be 1.0 for very small negative x.
+ // Clamp against the value just below 1.0.
+ return Min(frc, As<Float4>(Int4(0x3F7FFFFF)));
}
RValue<Float4> Floor(RValue<Float4> x)
