[HLSL] [DirectX] translate llvm fast math flags to llvm 3.7 fast math flags

[spall]

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

[llvmbot]

@llvm/pr-subscribers-backend-directx

Author: Sarah Spall (spall)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/122025.diff

2 Files Affected:

• (modified) llvm/lib/Target/DirectX/DXILWriter/DXILBitcodeWriter.cpp (+2-6)
• (added) llvm/test/tools/dxil-dis/fastmath.ll (+23)

diff --git a/llvm/lib/Target/DirectX/DXILWriter/DXILBitcodeWriter.cpp b/llvm/lib/Target/DirectX/DXILWriter/DXILBitcodeWriter.cpp
index 45aadac861946b..be68d46a876db2 100644
--- a/llvm/lib/Target/DirectX/DXILWriter/DXILBitcodeWriter.cpp
+++ b/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;
diff --git a/llvm/test/tools/dxil-dis/fastmath.ll b/llvm/test/tools/dxil-dis/fastmath.ll
new file mode 100644
index 00000000000000..7f4ba5b4cdd9f8
--- /dev/null
+++ b/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" }
+

[bfavela]

Why is "approx func" removed?

[spall]

https://releases.llvm.org/3.7.1/docs/LangRef.html#fast-math-flags
Not listed in fast math flags for 3.7, is this an error?

[bfavela]

Ah, good point... I don't want to stand in the way of this getting in, but neither is "AllowContract". Should we be asserting on these instead? (it seems "UnsafeAlgebra" was just renamed to "AllowReassoc")
https://github.com/microsoft/DirectXShaderCompiler/blob/070d0d5a2beacef9eeb51037a9b04665716fd6f3/include/llvm/IR/Operator.h#L171

[spall]

@llvm-beanz thoughts on this?

[llvm-beanz]

ApproxFunc has no equivalent in LLVM 3.7 so there is no bitcode representation of it. We have no choice but to drop it.

[llvm-beanz]

Also, in LLVM 3.7 UnsafeAlgebra was a magic catch all flag for "all things fast", it was separated out into granular flags in this commit - 629c41153876b.

It had the impact in the LLVM 3.7 optimizer of controlling both contraction and reassociation (to the extent that LLVM 3.7 had optimizations).