[llvm] [AMDGPU] - Add constant folding for s_bitreplicate (PR #72366)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: Jessica Del (OutOfCache)

<details>
<summary>Changes</summary>

If the input is a constant, we can constant fold the s_bitreplicate operation.

---
Full diff: https://github.com/llvm/llvm-project/pull/72366.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/ConstantFolding.cpp (+18) 
- (modified) llvm/test/CodeGen/AMDGPU/llvm.amdgcn.bitreplicate.ll (+2-2) 


``````````diff
diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 966a65ac26b8017..e401efa6c67c2da 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -1533,6 +1533,7 @@ bool llvm::canConstantFoldCallTo(const CallBase *Call, const Function *F) {
   case Intrinsic::amdgcn_perm:
   case Intrinsic::amdgcn_wave_reduce_umin:
   case Intrinsic::amdgcn_wave_reduce_umax:
+  case Intrinsic::amdgcn_s_bitreplicate:
   case Intrinsic::arm_mve_vctp8:
   case Intrinsic::arm_mve_vctp16:
   case Intrinsic::arm_mve_vctp32:
@@ -2422,6 +2423,23 @@ static Constant *ConstantFoldScalarCall1(StringRef Name,
 
       return ConstantFP::get(Ty->getContext(), Val);
     }
+
+    case Intrinsic::amdgcn_s_bitreplicate: {
+      uint64_t Val = Op->getZExtValue();
+      uint64_t ReplicatedVal = 0;
+      uint64_t ReplicatedOnes = 0b11;
+      // Input operand is always b32
+      for (unsigned i = 0; i < 32; ++i, ReplicatedOnes <<= 2, Val >>= 1) {
+        uint64_t Bit = Val & 1;
+
+        if (!Bit)
+          continue;
+
+        ReplicatedVal |= ReplicatedOnes;
+      }
+      return ConstantInt::get(Ty, ReplicatedVal);
+    }
+
     default:
       return nullptr;
     }
diff --git a/llvm/test/CodeGen/AMDGPU/llvm.amdgcn.bitreplicate.ll b/llvm/test/CodeGen/AMDGPU/llvm.amdgcn.bitreplicate.ll
index 027c9ef5e7cc349..0937280e6352c80 100644
--- a/llvm/test/CodeGen/AMDGPU/llvm.amdgcn.bitreplicate.ll
+++ b/llvm/test/CodeGen/AMDGPU/llvm.amdgcn.bitreplicate.ll
@@ -8,8 +8,8 @@ define i64 @test_s_bitreplicate_constant() {
 ; GFX11-LABEL: test_s_bitreplicate_constant:
 ; GFX11:       ; %bb.0: ; %entry
 ; GFX11-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
-; GFX11-NEXT:    s_bitreplicate_b64_b32 s[0:1], 0x85fe3a92
-; GFX11-NEXT:    v_dual_mov_b32 v0, s0 :: v_dual_mov_b32 v1, s1
+; GFX11-NEXT:    v_mov_b32_e32 v0, 0xfccc30c
+; GFX11-NEXT:    v_mov_b32_e32 v1, 0xc033fffc
 ; GFX11-NEXT:    s_setpc_b64 s[30:31]
 entry:
   %br = call i64 @llvm.amdgcn.s.bitreplicate(i32 u0x85FE3A92)

``````````

</details>


https://github.com/llvm/llvm-project/pull/72366