[llvm] [RISCV] Add isel optimization for (add x, (and (sra y, c2), c1)) to recover regression from #101751. (PR #104114)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Craig Topper (topperc)

<details>
<summary>Changes</summary>

If c1 is a shifted mask with c3 leading zeros and c4 trailing zeros. If c2 is greater than c3, we can use (srli (srai y, c2 - c3), c3 + c4) followed by a SHXADD with c4 as the X ammount.

I'll also improve the non-Zba case in a follow up patch.

---
Full diff: https://github.com/llvm/llvm-project/pull/104114.diff


2 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp (+27) 
- (modified) llvm/test/CodeGen/RISCV/rv64zba.ll (+23) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp b/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
index 35681c620eed5..bcc719737b405 100644
--- a/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
@@ -3019,6 +3019,33 @@ bool RISCVDAGToDAGISel::selectSHXADDOp(SDValue N, unsigned ShAmt,
           return true;
         }
       }
+    } else if (N0.getOpcode() == ISD::SRA && N0.hasOneUse() &&
+               isa<ConstantSDNode>(N.getOperand(1))) {
+      uint64_t Mask = N.getConstantOperandVal(1);
+      unsigned C2 = N0.getConstantOperandVal(1);
+
+      // Look for (and (sra y, c2), c1) where c1 is a shifted mask with c3
+      // leading zeros and c4 trailing zeros. If c2 is greater than c3, we can
+      // use (srli (srai y, c2 - c3), c3 + c4) followed by a SHXADD with c4 as
+      // the X ammount.
+      if (isShiftedMask_64(Mask)) {
+        unsigned XLen = Subtarget->getXLen();
+        unsigned Leading = XLen - llvm::bit_width(Mask);
+        unsigned Trailing = llvm::countr_zero(Mask);
+        if (C2 > Leading && Trailing == ShAmt) {
+          SDLoc DL(N);
+          EVT VT = N.getValueType();
+          Val = SDValue(CurDAG->getMachineNode(
+                            RISCV::SRAI, DL, VT, N0.getOperand(0),
+                            CurDAG->getTargetConstant(C2 - Leading, DL, VT)),
+                        0);
+          Val = SDValue(CurDAG->getMachineNode(
+                            RISCV::SRLI, DL, VT, Val,
+                            CurDAG->getTargetConstant(Leading + ShAmt, DL, VT)),
+                        0);
+          return true;
+        }
+      }
     }
   } else if (bool LeftShift = N.getOpcode() == ISD::SHL;
              (LeftShift || N.getOpcode() == ISD::SRL) &&
diff --git a/llvm/test/CodeGen/RISCV/rv64zba.ll b/llvm/test/CodeGen/RISCV/rv64zba.ll
index 87796e2c7b72e..b1b4707265d7f 100644
--- a/llvm/test/CodeGen/RISCV/rv64zba.ll
+++ b/llvm/test/CodeGen/RISCV/rv64zba.ll
@@ -2988,3 +2988,26 @@ entry:
   %2 = and i64 %1, 34359738360
   ret i64 %2
 }
+
+define ptr @srai_srli_sh3add(ptr %0, i64 %1) nounwind {
+; RV64I-LABEL: srai_srli_sh3add:
+; RV64I:       # %bb.0: # %entry
+; RV64I-NEXT:    srai a1, a1, 35
+; RV64I-NEXT:    li a2, -57
+; RV64I-NEXT:    srli a2, a2, 3
+; RV64I-NEXT:    and a1, a1, a2
+; RV64I-NEXT:    add a0, a0, a1
+; RV64I-NEXT:    ret
+;
+; RV64ZBA-LABEL: srai_srli_sh3add:
+; RV64ZBA:       # %bb.0: # %entry
+; RV64ZBA-NEXT:    srai a1, a1, 32
+; RV64ZBA-NEXT:    srli a1, a1, 6
+; RV64ZBA-NEXT:    sh3add a0, a1, a0
+; RV64ZBA-NEXT:    ret
+entry:
+  %2 = ashr i64 %1, 32
+  %3 = lshr i64 %2, 6
+  %4 = getelementptr i64, ptr %0, i64 %3
+  ret ptr %4
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/104114