[RISCV] Improve worklist management in the DAG combine for SLLW/SRLW/SRAW

This combine makes two calls to SimplifyDemandedBits, one for the LHS and one
for the RHS. If the LHS call returns true, we don't make the RHS call. When
SimplifyDemandedBits makes a change, it will add the nodes around the change to
the DAG combiner worklist. If the simplification happens on the first recursion
step, the N will get added to the worklist. But if the simplification happens
deeper in the recursion, then N will not be revisited until the next time the
DAG combiner runs.

This patch explicitly addes N to the worklist anytime a Simplification is made.
Without this we might miss additional simplifications on the LHS or never
simplify the RHS. Special care also needs to be taken to not add N if it has
been CSEd by the simplification. There are similar examples in DAGCombiner and
the X86 target, but I don't have a test for it for RISC-V. I've also returned
SDValue(N, 0) instead of SDValue() so DAGCombiner knows a change was made and
will update its Statistic variable.

The test here was constructed so that 2 simplifications happen to the LHS.
Without this fix one happens in the post type legalization DAG combine and the
other happens after LegalizeDAG. This prevents the RHS from ever being
simplified causing the left and right shift to clear the upper 32 bits of the
RHS to be left behind.

Differential Revision: https://reviews.llvm.org/D90339

diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 773a098..5916bb3 100644 (file)
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -1071,9 +1071,12 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
     SDValue RHS = N->getOperand(1);
     APInt LHSMask = APInt::getLowBitsSet(LHS.getValueSizeInBits(), 32);
     APInt RHSMask = APInt::getLowBitsSet(RHS.getValueSizeInBits(), 5);
-    if ((SimplifyDemandedBits(N->getOperand(0), LHSMask, DCI)) ||
-        (SimplifyDemandedBits(N->getOperand(1), RHSMask, DCI)))
-      return SDValue();
+    if (SimplifyDemandedBits(N->getOperand(0), LHSMask, DCI) ||
+        SimplifyDemandedBits(N->getOperand(1), RHSMask, DCI)) {
+      if (N->getOpcode() != ISD::DELETED_NODE)
+        DCI.AddToWorklist(N);
+      return SDValue(N, 0);
+    }
     break;
   }
   case RISCVISD::FMV_X_ANYEXTW_RV64: {

diff --git a/llvm/test/CodeGen/RISCV/rv64i-demanded-bits.ll b/llvm/test/CodeGen/RISCV/rv64i-demanded-bits.ll
index 319270c..671f90b 100644 (file)
--- a/llvm/test/CodeGen/RISCV/rv64i-demanded-bits.ll
+++ b/llvm/test/CodeGen/RISCV/rv64i-demanded-bits.ll
@@ -14,8 +14,6 @@ define i32 @foo(i32 %x, i32 %y, i32 %z) {
 ; CHECK-NEXT:    mul a0, a0, a0
 ; CHECK-NEXT:    add a0, a0, a2
 ; CHECK-NEXT:    addi a0, a0, 1
-; CHECK-NEXT:    slli a1, a1, 32
-; CHECK-NEXT:    srli a1, a1, 32
 ; CHECK-NEXT:    sllw a0, a0, a1
 ; CHECK-NEXT:    ret
   %b = mul i32 %x, %x