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

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -21470,8 +21470,16 @@ 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.
+    // TODO: A more precise answer could be calculated depending on known bits
+    // in the shift amount.
+    return std::max(Tmp, 33U);
+  }
   case RISCVISD::SLLW:
-  case RISCVISD::SRAW:
   case RISCVISD::SRLW:
   case RISCVISD::DIVW:
   case RISCVISD::DIVUW:
@@ -21482,9 +21490,7 @@ unsigned RISCVTargetLowering::ComputeNumSignBitsForTargetNode(
   case RISCVISD::FCVT_WU_RV64:
   case RISCVISD::STRICT_FCVT_W_RV64:
   case RISCVISD::STRICT_FCVT_WU_RV64:
-    // TODO: As the result is sign-extended, this is conservatively correct. A
-    // more precise answer could be calculated for SRAW depending on known
-    // bits in the shift amount.
+    // TODO: As the result is sign-extended, this is conservatively correct.
     return 33;
   case RISCVISD::VMV_X_S: {
     // The number of sign bits of the scalar result is computed by obtaining the

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
+}