From 2dc262fe23389f6b916a86d37624acd3789b7712 Mon Sep 17 00:00:00 2001
From: Rex Xu <rex.xu@amd.com>
Date: Wed, 8 Nov 2023 15:37:44 +0800
Subject: [PATCH] Fix issues of fract(x)

The problem is similar to modf(x) when x=-0.0 or +INF/-INF. Although
SPIR-V spec says nothing about such cases, the OpenCL spec does define
the results of those operations as follow:

  fract(-0.0) = -0.0
  fract(+INF) = 0.0
  fract(-INF) = -0.0

Hence, we follow what have been done for modf.
---
 lgc/builder/ArithBuilder.cpp | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/lgc/builder/ArithBuilder.cpp b/lgc/builder/ArithBuilder.cpp
index 69dc6e41d6..a8935a3dd7 100644
--- a/lgc/builder/ArithBuilder.cpp
+++ b/lgc/builder/ArithBuilder.cpp
@@ -700,6 +700,32 @@ Value *BuilderImpl::CreateSSign(Value *x, const Twine &instName) {
 Value *BuilderImpl::CreateFract(Value *x, const Twine &instName) {
   // We need to scalarize this ourselves.
   Value *result = scalarize(x, [this](Value *x) { return CreateIntrinsic(Intrinsic::amdgcn_fract, x->getType(), x); });
+
+  // NOTE: Although SPIR-V spec says nothing about such cases: fract(-0.0), fract(+INF), fract(-INF), OpenCL spec does
+  // have following defintions:
+  //
+  //  fract(-0.0) = -0.0
+  //  fract(+INF) = +0.0
+  //  fract(-INF) = -0.0
+  //
+  // When we follow it, we have two issues that are similar to modf(x):
+  //
+  //   1. When we input x=+INF/-INF to above formula, we finally get the computation of (-INF) + INF or INF - INF.
+  //      The result is NaN returned by HW.
+  //   2. When we input x=-0.0 to above formula, we finally get the addition of (-0.0) + 0.0. The result is +0.0
+  //      returned by HW.
+  //
+  // Hence, we have to manually check those special cases:
+  //
+  //   y = fract(x)
+  //   y = x == -0.0 || x == INF ? copysign(0.0, x) : y, when either NSZ or NoInfs is not present
+  if (!getFastMathFlags().noSignedZeros() || !getFastMathFlags().noInfs()) {
+    Value *isNegZeroOrInf =
+        createIsFPClass(x, CmpClass::NegativeZero | CmpClass::NegativeInfinity | CmpClass::PositiveInfinity);
+    Value *signedZero = CreateCopySign(ConstantFP::getNullValue(x->getType()), x);
+    result = CreateSelect(isNegZeroOrInf, signedZero, result);
+  }
+
   result->setName(instName);
   return result;
 }