[HLSL] [DirectX] translate llvm fast math flags to llvm 3.7 fast math flags (llvm#122025)

Translate modern LLVM fast math flags to LLVM 3.7 equivalent in DXIL
bitcode. Mostly use patch from llvm#120630
Closes llvm#120630

Author: spall

llvm/lib/Target/DirectX/DXILWriter/DXILBitcodeWriter.cpp

@@ -749,8 +749,8 @@ uint64_t DXILBitcodeWriter::getOptimizationFlags(const Value *V) {
     if (PEO->isExact())
       Flags |= 1 << bitc::PEO_EXACT;
   } else if (const auto *FPMO = dyn_cast<FPMathOperator>(V)) {
-    if (FPMO->hasAllowReassoc())
-      Flags |= bitc::AllowReassoc;
+    if (FPMO->hasAllowReassoc() || FPMO->hasAllowContract())
+      Flags |= bitc::UnsafeAlgebra;
     if (FPMO->hasNoNaNs())
       Flags |= bitc::NoNaNs;
     if (FPMO->hasNoInfs())
@@ -759,10 +759,6 @@ uint64_t DXILBitcodeWriter::getOptimizationFlags(const Value *V) {
       Flags |= bitc::NoSignedZeros;
     if (FPMO->hasAllowReciprocal())
       Flags |= bitc::AllowReciprocal;
-    if (FPMO->hasAllowContract())
-      Flags |= bitc::AllowContract;
-    if (FPMO->hasApproxFunc())
-      Flags |= bitc::ApproxFunc;
   }
 
   return Flags;

llvm/test/tools/dxil-dis/fastmath.ll

@@ -0,0 +1,23 @@
+; RUN: llc %s --filetype=obj -o - | dxil-dis -o - | FileCheck %s
+target triple = "dxil-unknown-shadermodel6.7-library"
+
+define float @fma(float %0, float %1, float %2) #0 {
+  ; verify reassoc and contract are converted to fast
+  ; CHECK: %4 = fmul fast float %0, %1
+  %4 = fmul reassoc float %0, %1
+  ; CHECK-NEXT: %5 = fadd fast float %4, %2
+  %5 = fadd contract float %4, %2
+  ; verify these are converted to a single fast flag
+  ; CHECK-NEXT: %6 = fmul fast float %0, %1
+  %6 = fmul reassoc contract float %0, %1
+  ; verify these flags are maintained
+  ; CHECK-NEXT: %7 = fadd nnan ninf nsz arcp float %0, %1
+  %7 = fadd nnan ninf nsz arcp float %0, %1
+  ; verify that afn is removed
+  ; CHECK-NEXT: %8 = fmul float %0, %1
+  %8 = fmul afn float %0, %1
+  ret float %5
+}
+
+attributes #0 = { norecurse nounwind readnone willreturn "disable-tail-calls"="false" "waveops-include-helper-lanes" "fp32-denorm-mode"="any" "hlsl.export" }
+