[AMDGPU] InstCombine llvm.amdgcn.ds.bpermute with uniform arguments (#129895)

Author: jayfoad

llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp

@@ -1118,9 +1118,11 @@ GCNTTIImpl::instCombineIntrinsic(InstCombiner &IC, IntrinsicInst &II) const {
   }
   case Intrinsic::amdgcn_permlane64:
   case Intrinsic::amdgcn_readfirstlane:
-  case Intrinsic::amdgcn_readlane: {
-    // If the first argument is uniform these intrinsics return it unchanged.
-    const Use &Src = II.getArgOperandUse(0);
+  case Intrinsic::amdgcn_readlane:
+  case Intrinsic::amdgcn_ds_bpermute: {
+    // If the data argument is uniform these intrinsics return it unchanged.
+    unsigned SrcIdx = IID == Intrinsic::amdgcn_ds_bpermute ? 1 : 0;
+    const Use &Src = II.getArgOperandUse(SrcIdx);
     if (isTriviallyUniform(Src))
       return IC.replaceInstUsesWith(II, Src.get());
 
@@ -1152,6 +1154,22 @@ GCNTTIImpl::instCombineIntrinsic(InstCombiner &IC, IntrinsicInst &II) const {
       }
     }
 
+    // If the lane argument of bpermute is uniform, change it to readlane. This
+    // generates better code and can enable further optimizations because
+    // readlane is AlwaysUniform.
+    if (IID == Intrinsic::amdgcn_ds_bpermute) {
+      const Use &Lane = II.getArgOperandUse(0);
+      if (isTriviallyUniform(Lane)) {
+        Value *NewLane = IC.Builder.CreateLShr(Lane, 2);
+        Function *NewDecl = Intrinsic::getOrInsertDeclaration(
+            II.getModule(), Intrinsic::amdgcn_readlane, II.getType());
+        II.setCalledFunction(NewDecl);
+        II.setOperand(0, Src);
+        II.setOperand(1, NewLane);
+        return ⅈ
+      }
+    }
+
     return std::nullopt;
   }
   case Intrinsic::amdgcn_writelane: {

llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-intrinsics.ll

@@ -6583,3 +6583,42 @@ define i32 @prng_poison_i32() {
   %prng = call i32 @llvm.amdgcn.prng.b32(i32 poison)
   ret i32 %prng
 }
+
+; --------------------------------------------------------------------
+; llvm.amdgcn.ds.bpermute
+; --------------------------------------------------------------------
+
+define amdgpu_kernel void @ds_bpermute_uniform_src(ptr addrspace(1) %out, i32 %lane) {
+; CHECK-LABEL: @ds_bpermute_uniform_src(
+; CHECK-NEXT:    store i32 7, ptr addrspace(1) [[OUT:%.*]], align 4
+; CHECK-NEXT:    ret void
+;
+  %v = call i32 @llvm.amdgcn.ds.bpermute(i32 %lane, i32 7)
+  store i32 %v, ptr addrspace(1) %out
+  ret void
+}
+
+define amdgpu_kernel void @ds_bpermute_constant_lane(ptr addrspace(1) %out, i32 %src) {
+; CHECK-LABEL: @ds_bpermute_constant_lane(
+; CHECK-NEXT:    [[V:%.*]] = call i32 @llvm.amdgcn.readlane.i32(i32 [[SRC:%.*]], i32 7)
+; CHECK-NEXT:    store i32 [[V]], ptr addrspace(1) [[OUT:%.*]], align 4
+; CHECK-NEXT:    ret void
+;
+  %v = call i32 @llvm.amdgcn.ds.bpermute(i32 28, i32 %src)
+  store i32 %v, ptr addrspace(1) %out
+  ret void
+}
+
+define amdgpu_kernel void @ds_bpermute_uniform_lane(ptr addrspace(1) %out, i32 %lanearg, i32 %src) {
+; CHECK-LABEL: @ds_bpermute_uniform_lane(
+; CHECK-NEXT:    [[LANE:%.*]] = call i32 @llvm.amdgcn.readfirstlane.i32(i32 [[LANEARG:%.*]])
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr i32 [[LANE]], 2
+; CHECK-NEXT:    [[V:%.*]] = call i32 @llvm.amdgcn.readlane.i32(i32 [[SRC:%.*]], i32 [[TMP1]])
+; CHECK-NEXT:    store i32 [[V]], ptr addrspace(1) [[OUT:%.*]], align 4
+; CHECK-NEXT:    ret void
+;
+  %lane = call i32 @llvm.amdgcn.readfirstlane(i32 %lanearg)
+  %v = call i32 @llvm.amdgcn.ds.bpermute(i32 %lane, i32 %src)
+  store i32 %v, ptr addrspace(1) %out
+  ret void
+}