[llvm] [RISCV] Add SRAW to ComputeNumSignBitsForTargetNode. (PR #155564)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-risc-v

Author: Craig Topper (topperc)

<details>
<summary>Changes</summary>



---
Full diff: https://github.com/llvm/llvm-project/pull/155564.diff


2 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVISelLowering.cpp (+7-1) 
- (modified) llvm/test/CodeGen/RISCV/rv64i-shift-sext.ll (+15) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index b84bd1ce0ac50..c0e3a18c1096a 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -21451,8 +21451,14 @@ unsigned RISCVTargetLowering::ComputeNumSignBitsForTargetNode(
     if (Tmp < 33) return 1;
     return 33;
   }
+  case RISCVISD::SRAW: {
+    unsigned Tmp =
+        DAG.ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth + 1);
+    // sraw produces at least 33 sign bits. If the input already has more than
+    // 33 sign bits sraw, will preserve them.
+    return std::max(Tmp, 33U);
+  }
   case RISCVISD::SLLW:
-  case RISCVISD::SRAW:
   case RISCVISD::SRLW:
   case RISCVISD::DIVW:
   case RISCVISD::DIVUW:
diff --git a/llvm/test/CodeGen/RISCV/rv64i-shift-sext.ll b/llvm/test/CodeGen/RISCV/rv64i-shift-sext.ll
index b3c22a5322cb4..90735d88494b5 100644
--- a/llvm/test/CodeGen/RISCV/rv64i-shift-sext.ll
+++ b/llvm/test/CodeGen/RISCV/rv64i-shift-sext.ll
@@ -220,3 +220,18 @@ define signext i32 @test14(ptr %0, ptr %1, i64 %2) {
   %12 = add i32 %9, %11
   ret i32 %12
 }
+
+; Test that we can propagate sign bits through sraw. We should use an slli
+; instead of slliw.
+define signext i32 @test15(i32 signext %x, i32 signext %y) {
+; RV64I-LABEL: test15:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    srli a0, a0, 1
+; RV64I-NEXT:    sraw a0, a0, a1
+; RV64I-NEXT:    slli a0, a0, 1
+; RV64I-NEXT:    ret
+  %a = ashr i32 %x, 1
+  %b = ashr i32 %a, %y
+  %c = shl i32 %b, 1
+  ret i32 %c
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/155564