[clang] [AMDGPU] Lower __builtin_read_exec_hi to use amdgcn_ballot (PR #69567)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-codegen

Author: Rana Pratap Reddy (ranapratap55)

<details>
<summary>Changes</summary>

Currently __builtin_read_exec_hi lowers to llvm.read_register, this patch lowers it to use amdgcn_ballot.

---
Full diff: https://github.com/llvm/llvm-project/pull/69567.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGBuiltin.cpp (+20-7) 
- (modified) clang/test/CodeGenOpenCL/builtins-amdgcn.cl (+3-1) 


``````````diff
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index db9f354fa8386d3..d60826f293f0c46 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -7997,14 +7997,26 @@ enum SpecialRegisterAccessKind {
 
 static Value *EmitAMDGCNBallotForExec(CodeGenFunction &CGF, const CallExpr *E,
                                       llvm::Type *RegisterType,
-                                      llvm::Type *ValueType) {
+                                      llvm::Type *ValueType, bool isExecHi) {
   CodeGen::CGBuilderTy &Builder = CGF.Builder;
   CodeGen::CodeGenModule &CGM = CGF.CGM;
 
   llvm::Type *ResultType = CGF.ConvertType(E->getType());
-  Function *F = CGM.getIntrinsic(Intrinsic::amdgcn_ballot, {ResultType});
-  llvm::Value *Call = Builder.CreateCall(F, {Builder.getInt1(true)});
-  return Call;
+  llvm::Value *Call;
+  Function *F;
+  
+  if (isExecHi) {
+    F = CGM.getIntrinsic(Intrinsic::amdgcn_ballot, {RegisterType});
+    Call = Builder.CreateCall(F, {Builder.getInt1(true)});
+    Value *C1 = llvm::ConstantInt::get(ValueType, 32);
+    Value *Rt2 = Builder.CreateLShr(Call, C1);
+    Rt2 = Builder.CreateTruncOrBitCast(Rt2, CGF.Int32Ty);
+    return Rt2;
+  } else {
+    F = CGM.getIntrinsic(Intrinsic::amdgcn_ballot, {ResultType});
+    Call = Builder.CreateCall(F, {Builder.getInt1(true)});
+    return Call;
+  }
 }
 
 // Generates the IR for the read/write special register builtin,
@@ -17837,10 +17849,11 @@ Value *CodeGenFunction::EmitAMDGPUBuiltinExpr(unsigned BuiltinID,
     return Builder.CreateCall(F, {Addr, Val, ZeroI32, ZeroI32, ZeroI1});
   }
   case AMDGPU::BI__builtin_amdgcn_read_exec:
+    return EmitAMDGCNBallotForExec(*this, E, Int64Ty, Int64Ty, false);
   case AMDGPU::BI__builtin_amdgcn_read_exec_lo:
-  case AMDGPU::BI__builtin_amdgcn_read_exec_hi: {
-    return EmitAMDGCNBallotForExec(*this, E, Int64Ty, Int64Ty);
-  }
+    return EmitAMDGCNBallotForExec(*this, E, Int32Ty, Int32Ty, false);
+  case AMDGPU::BI__builtin_amdgcn_read_exec_hi:
+    return EmitAMDGCNBallotForExec(*this, E, Int64Ty, Int64Ty, true);
   case AMDGPU::BI__builtin_amdgcn_image_bvh_intersect_ray:
   case AMDGPU::BI__builtin_amdgcn_image_bvh_intersect_ray_h:
   case AMDGPU::BI__builtin_amdgcn_image_bvh_intersect_ray_l:
diff --git a/clang/test/CodeGenOpenCL/builtins-amdgcn.cl b/clang/test/CodeGenOpenCL/builtins-amdgcn.cl
index 8938642e3b19f8c..0bc9a54682d3e31 100644
--- a/clang/test/CodeGenOpenCL/builtins-amdgcn.cl
+++ b/clang/test/CodeGenOpenCL/builtins-amdgcn.cl
@@ -526,7 +526,9 @@ void test_read_exec_lo(global uint* out) {
 // CHECK: declare i32 @llvm.amdgcn.ballot.i32(i1) #[[$NOUNWIND_READONLY:[0-9]+]]
 
 // CHECK-LABEL: @test_read_exec_hi(
-// CHECK: call i32 @llvm.amdgcn.ballot.i32(i1 true)
+// CHECK: call i64 @llvm.amdgcn.ballot.i64(i1 true)
+// CHECK: lshr i64 [[A:%.*]], 32
+// CHECK: trunc i64 [[B:%.*]] to i32
 void test_read_exec_hi(global uint* out) {
   *out = __builtin_amdgcn_read_exec_hi();
 }

``````````

</details>


https://github.com/llvm/llvm-project/pull/69567