[DirectX] Implement UseNativeLowPrecision shader flag analysis #134288

Icohedron · 2025-04-03T18:17:07Z

Implement the shader flag analysis to set the UseNativeLowPrecision DXIL module flag.

The flag is only able to be set when the command-line flag -enable-16bit-types is passed to clang-dxc, or equivalently -fnative-half-type is passed to clang.
When the command-line flag is passed, a module metadata flag called "dx.nativelowprec" is set to 1.
The DXILShaderFlags shader flags analysis checks that the module metadata flag "dx.nativelowprec" is set to 1 and the DXIL Version is 1.2 or greater before setting the UseNativeLowPrecision DXIL module flag.

llvmbot · 2025-04-03T18:17:46Z

@llvm/pr-subscribers-clang
@llvm/pr-subscribers-backend-directx

@llvm/pr-subscribers-clang-codegen

Author: Deric C. (Icohedron)

Changes

Fixes #112267

Full diff: https://github.com/llvm/llvm-project/pull/134288.diff

4 Files Affected:

(modified) clang/lib/CodeGen/CGHLSLRuntime.cpp (+3)
(added) clang/test/CodeGenHLSL/enable-16bit-types.hlsl (+9)
(modified) llvm/lib/Target/DirectX/DXILShaderFlags.cpp (+6)
(added) llvm/test/CodeGen/DirectX/ShaderFlags/use-native-low-precision.ll (+45)

diff --git a/clang/lib/CodeGen/CGHLSLRuntime.cpp b/clang/lib/CodeGen/CGHLSLRuntime.cpp
index 3b1810b62a2cd..ca5f180d2a64c 100644
--- a/clang/lib/CodeGen/CGHLSLRuntime.cpp
+++ b/clang/lib/CodeGen/CGHLSLRuntime.cpp
@@ -282,10 +282,13 @@ void CGHLSLRuntime::addHLSLBufferLayoutType(const RecordType *StructType,
 
 void CGHLSLRuntime::finishCodeGen() {
   auto &TargetOpts = CGM.getTarget().getTargetOpts();
+  auto &LangOpts = CGM.getLangOpts();
   llvm::Module &M = CGM.getModule();
   Triple T(M.getTargetTriple());
   if (T.getArch() == Triple::ArchType::dxil)
     addDxilValVersion(TargetOpts.DxilValidatorVersion, M);
+  if (LangOpts.NativeHalfType)
+    M.setModuleFlag(llvm::Module::ModFlagBehavior::Error, "dx.nativelowprec", 1);
 
   generateGlobalCtorDtorCalls();
 }
diff --git a/clang/test/CodeGenHLSL/enable-16bit-types.hlsl b/clang/test/CodeGenHLSL/enable-16bit-types.hlsl
new file mode 100644
index 0000000000000..7da66a3ed863b
--- /dev/null
+++ b/clang/test/CodeGenHLSL/enable-16bit-types.hlsl
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fnative-half-type -finclude-default-header -triple dxil-pc-shadermodel6.3-library -emit-llvm -disable-llvm-passes -o - %s | FileCheck %s --check-prefix=FLAG
+// RUN: %clang_cc1 -finclude-default-header -triple dxil-pc-shadermodel6.3-library -emit-llvm -disable-llvm-passes -o - %s | FileCheck %s --check-prefix=NOFLAG
+
+// NOTE: -enable-16bit-types is a DXCFlag that aliases -fnative-half-type
+
+// FLAG-DAG: ![[NLP:.*]] = !{i32 1, !"dx.nativelowprec", i32 1}
+// FLAG-DAG: !llvm.module.flags = !{{{.*}}![[NLP]]{{.*}}}
+
+// NOFLAG-NOT: dx.nativelowprec
diff --git a/llvm/lib/Target/DirectX/DXILShaderFlags.cpp b/llvm/lib/Target/DirectX/DXILShaderFlags.cpp
index babf495220393..437d1402ccedc 100644
--- a/llvm/lib/Target/DirectX/DXILShaderFlags.cpp
+++ b/llvm/lib/Target/DirectX/DXILShaderFlags.cpp
@@ -188,6 +188,12 @@ void ModuleShaderFlags::initialize(Module &M, DXILResourceTypeMap &DRTM,
         continue;
       }
 
+      // Set UseNativeLowPrecision using dx.nativelowprec module metadata
+      if (auto *NativeLowPrec = mdconst::extract_or_null<ConstantInt>(
+              M.getModuleFlag("dx.nativelowprec")))
+        if (NativeLowPrec->getValue() != 0)
+          SCCSF.UseNativeLowPrecision = true;
+
       ComputedShaderFlags CSF;
       for (const auto &BB : *F)
         for (const auto &I : BB)
diff --git a/llvm/test/CodeGen/DirectX/ShaderFlags/use-native-low-precision.ll b/llvm/test/CodeGen/DirectX/ShaderFlags/use-native-low-precision.ll
new file mode 100644
index 0000000000000..95c916b169cdf
--- /dev/null
+++ b/llvm/test/CodeGen/DirectX/ShaderFlags/use-native-low-precision.ll
@@ -0,0 +1,45 @@
+; RUN: opt -S --passes="print-dx-shader-flags" 2>&1 %s | FileCheck %s
+; RUN: llc %s --filetype=obj -o - | obj2yaml | FileCheck %s --check-prefix=DXC
+
+target triple = "dxil-pc-shadermodel6.7-library"
+
+;CHECK: ; Combined Shader Flags for Module
+;CHECK-NEXT: ; Shader Flags Value: 0x00800020
+;CHECK-NEXT: ;
+;CHECK-NEXT: ; Note: shader requires additional functionality:
+;CHECK-NEXT: ; Note: extra DXIL module flags:
+;CHECK-NEXT: ;       D3D11_1_SB_GLOBAL_FLAG_ENABLE_MINIMUM_PRECISION
+;CHECK-NEXT: ;       Native 16bit types enabled
+;CHECK-NEXT: ;
+;CHECK-NEXT: ; Shader Flags for Module Functions
+
+;CHECK-LABEL: ; Function add_i16 : 0x00800020
+define i16 @add_i16(i16 %a, i16 %b) #0 {
+  %sum = add i16 %a, %b
+  ret i16 %sum
+}
+
+; NOTE: The flag for native low precision is set for every function in the
+; module regardless of whether or not the function uses low precision data
+; types. This matches the behavior in DXC
+;CHECK-LABEL: ; Function add_i32 : 0x00800000
+define i32 @add_i32(i32 %a, i32 %b) #0 {
+  %sum = add i32 %a, %b
+  ret i32 %sum
+}
+
+;CHECK-LABEL: ; Function add_half : 0x00800020
+define half @add_half(half %a, half %b) #0 {
+  %sum = fadd half %a, %b
+  ret half %sum
+}
+
+attributes #0 = { convergent norecurse nounwind "hlsl.export" }
+
+!llvm.module.flags = !{!0}
+!0 = !{i32 1, !"dx.nativelowprec", i32 1}
+
+; DXC: - Name:            SFI0
+; DXC-NEXT:     Size:            8
+; DXC-NOT:     Flags:
+; DXC: ...

github-actions · 2025-04-03T18:19:34Z

✅ With the latest revision this PR passed the C/C++ code formatter.

inbelic

Assuming -fnative-half-type can be directly used as an alias to enable-16bit-types in DXC, this LGTM. Just some nits otherwise

llvm/test/CodeGen/DirectX/ShaderFlags/use-native-low-precision.ll

clang/lib/CodeGen/CGHLSLRuntime.cpp

clang/test/CodeGenHLSL/enable-16bit-types.hlsl

llvm/test/CodeGen/DirectX/ShaderFlags/use-native-low-precision.ll

clang/test/CodeGenHLSL/enable-16bit-types.hlsl

farzonl · 2025-04-08T20:02:21Z

clang/test/CodeGenHLSL/enable-16bit-types.hlsl

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fnative-half-type -finclude-default-header -triple dxil-pc-shadermodel6.3-library -emit-llvm -disable-llvm-passes -o - %s | FileCheck %s --check-prefix=FLAG


NIT: For future cleanup and test organization could we move the Shader Flag tests into their own directory like the Builtins have their own directory? Not asking for this PR.

farzonl · 2025-04-08T20:09:24Z

llvm/lib/Target/DirectX/DXILShaderFlags.cpp

+      // Set UseNativeLowPrecision using dx.nativelowprec module metadata
+      if (auto *NativeLowPrec = mdconst::extract_or_null<ConstantInt>(
+              M.getModuleFlag("dx.nativelowprec")))
+        if (MMDI.DXILVersion >= VersionTuple(1, 2) &&


So I know the frontend likely has checks to confirm SM 6.2 or greater for this flag. If the backend is dxil-pc-shadermodel6.1 what would happen in a test case like use-native-low-precision.ll? Would we just expect to not see !0 = !{i32 1, !"dx.nativelowprec", i32 1}?

The frontend is what is adding !0 = !{i32 1, !"dx.nativelowprec", i32 1}, so we wouldn't see it in the module.

If it is manually added to a module that is SM 6.1 or older, then it should have no effect on the shader flags.
Should I add a test for this?

My concern was if the validator would allow the dxil if it was less than sm 6.2 and we saw nativelowprec set. The backend would be our last place to catch that and avoid the validator complaining.

If that isn't a concern then I don't think we need a test.

I don't think the validator interprets any module metadata flag named "dx.nativelowpres", so it would just be ignored if it is present in the DXIL even when the SM is lower than 6.2.

farzonl · 2025-04-08T20:11:21Z

Codgen looks right. The only changes I want to see are around testing.

…134288) Fixes llvm#112267 Implement the shader flag analysis to set the UseNativeLowPrecision DXIL module flag. The flag is only able to be set when the command-line flag `-enable-16bit-types` is passed to clang-dxc, or equivalently `-fnative-half-type` is passed to clang. When the command-line flag is passed, a module metadata flag called "dx.nativelowprec" is set to 1. The DXILShaderFlags shader flags analysis checks that the module metadata flag "dx.nativelowprec" is set to 1 and the DXIL Version is 1.2 or greater before setting the UseNativeLowPrecision DXIL module flag.

Icohedron added 2 commits April 3, 2025 16:23

Add test for UseNativeLowPrecision shader flag

c482c96

Implement UseNativeLowPrecision shader flag analysis

1d90753

llvmbot added clang Clang issues not falling into any other category clang:codegen IR generation bugs: mangling, exceptions, etc. backend:DirectX HLSL HLSL Language Support labels Apr 3, 2025

github-project-automation bot added this to HLSL Support Apr 3, 2025

Icohedron added 2 commits April 3, 2025 18:54

Add check for DXIL Version >= 1.2

33f5896

Apply clang-format

5656192

inbelic reviewed Apr 7, 2025

View reviewed changes

farzonl reviewed Apr 8, 2025

View reviewed changes

clang/test/CodeGenHLSL/enable-16bit-types.hlsl Outdated Show resolved Hide resolved

farzonl reviewed Apr 8, 2025

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Icohedron added 3 commits April 8, 2025 23:02

Replace clang_cc1 with clang_dxc in enable-16bit-types test

35a0804

Remove unnecessary attributes and obj2yml test

09864cf

Add a comment describing the purpose of the NativeHalfType conditional

0e8b7fa

farzonl approved these changes Apr 9, 2025

View reviewed changes

Add '-Xclang -emit-llvm' to avoid running the DirectX backend

71687e4

Icohedron merged commit 747d4a9 into llvm:main Apr 10, 2025
11 of 12 checks passed

damyanp moved this to Closed in HLSL Support Apr 25, 2025

damyanp removed this from HLSL Support Jun 25, 2025

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		@@ -0,0 +1,9 @@
		// RUN: %clang_cc1 -fnative-half-type -finclude-default-header -triple dxil-pc-shadermodel6.3-library -emit-llvm -disable-llvm-passes -o - %s \| FileCheck %s --check-prefix=FLAG

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