[DAGCombiner][RISCV] Optimize (zext nneg (truncate X)) if X has known sign bits. (#82227)

This treats the zext nneg as sext if X is known to have sufficient sign
bits to allow the zext or truncate or both to removed. This code is
taken from the same optimization for sext.

Author: topperc

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -13758,6 +13758,32 @@ SDValue DAGCombiner::visitZERO_EXTEND(SDNode *N) {
     EVT SrcVT = N0.getOperand(0).getValueType();
     EVT MinVT = N0.getValueType();
 
+    if (N->getFlags().hasNonNeg()) {
+      SDValue Op = N0.getOperand(0);
+      unsigned OpBits = SrcVT.getScalarSizeInBits();
+      unsigned MidBits = MinVT.getScalarSizeInBits();
+      unsigned DestBits = VT.getScalarSizeInBits();
+      unsigned NumSignBits = DAG.ComputeNumSignBits(Op);
+
+      if (OpBits == DestBits) {
+        // Op is i32, Mid is i8, and Dest is i32.  If Op has more than 24 sign
+        // bits, it is already ready.
+        if (NumSignBits > DestBits - MidBits)
+          return Op;
+      } else if (OpBits < DestBits) {
+        // Op is i32, Mid is i8, and Dest is i64.  If Op has more than 24 sign
+        // bits, just sext from i32.
+        // FIXME: This can probably be ZERO_EXTEND nneg?
+        if (NumSignBits > OpBits - MidBits)
+          return DAG.getNode(ISD::SIGN_EXTEND, DL, VT, Op);
+      } else {
+        // Op is i64, Mid is i8, and Dest is i32.  If Op has more than 56 sign
+        // bits, just truncate to i32.
+        if (NumSignBits > OpBits - MidBits)
+          return DAG.getNode(ISD::TRUNCATE, DL, VT, Op);
+      }
+    }
+
     // Try to mask before the extension to avoid having to generate a larger mask,
     // possibly over several sub-vectors.
     if (SrcVT.bitsLT(VT) && VT.isVector()) {

llvm/test/CodeGen/RISCV/sext-zext-trunc.ll

@@ -756,31 +756,18 @@ define void @zext_nneg_dominating_icmp_i64(i16 signext %0) {
 ; RV32I:       # %bb.0:
 ; RV32I-NEXT:    bltz a0, .LBB46_2
 ; RV32I-NEXT:  # %bb.1:
-; RV32I-NEXT:    slli a0, a0, 16
-; RV32I-NEXT:    srli a0, a0, 16
-; RV32I-NEXT:    li a1, 0
+; RV32I-NEXT:    srai a1, a0, 31
 ; RV32I-NEXT:    tail bar_i64
 ; RV32I-NEXT:  .LBB46_2:
 ; RV32I-NEXT:    ret
 ;
-; RV64I-LABEL: zext_nneg_dominating_icmp_i64:
-; RV64I:       # %bb.0:
-; RV64I-NEXT:    bltz a0, .LBB46_2
-; RV64I-NEXT:  # %bb.1:
-; RV64I-NEXT:    slli a0, a0, 48
-; RV64I-NEXT:    srli a0, a0, 48
-; RV64I-NEXT:    tail bar_i64
-; RV64I-NEXT:  .LBB46_2:
-; RV64I-NEXT:    ret
-;
-; RV64ZBB-LABEL: zext_nneg_dominating_icmp_i64:
-; RV64ZBB:       # %bb.0:
-; RV64ZBB-NEXT:    bltz a0, .LBB46_2
-; RV64ZBB-NEXT:  # %bb.1:
-; RV64ZBB-NEXT:    zext.h a0, a0
-; RV64ZBB-NEXT:    tail bar_i64
-; RV64ZBB-NEXT:  .LBB46_2:
-; RV64ZBB-NEXT:    ret
+; RV64-LABEL: zext_nneg_dominating_icmp_i64:
+; RV64:       # %bb.0:
+; RV64-NEXT:    bltz a0, .LBB46_2
+; RV64-NEXT:  # %bb.1:
+; RV64-NEXT:    tail bar_i64
+; RV64-NEXT:  .LBB46_2:
+; RV64-NEXT:    ret
   %2 = icmp sgt i16 %0, -1
   br i1 %2, label %3, label %5
 
@@ -800,8 +787,6 @@ define void @zext_nneg_dominating_icmp_i32(i16 signext %0) {
 ; RV32I:       # %bb.0:
 ; RV32I-NEXT:    bltz a0, .LBB47_2
 ; RV32I-NEXT:  # %bb.1:
-; RV32I-NEXT:    slli a0, a0, 16
-; RV32I-NEXT:    srli a0, a0, 16
 ; RV32I-NEXT:    tail bar_i32
 ; RV32I-NEXT:  .LBB47_2:
 ; RV32I-NEXT:    ret

llvm/test/CodeGen/VE/Scalar/ctlz.ll

@@ -34,7 +34,6 @@ define signext i32 @func32s(i32 signext %p) {
 ; CHECK-NEXT:    and %s0, %s0, (32)0
 ; CHECK-NEXT:    ldz %s0, %s0
 ; CHECK-NEXT:    lea %s0, -32(, %s0)
-; CHECK-NEXT:    and %s0, %s0, (32)0
 ; CHECK-NEXT:    b.l.t (, %s10)
   %r = tail call i32 @llvm.ctlz.i32(i32 %p, i1 true)
   ret i32 %r
@@ -202,7 +201,6 @@ define signext i32 @func32sx(i32 signext %p) {
 ; CHECK-NEXT:    and %s0, %s0, (32)0
 ; CHECK-NEXT:    ldz %s0, %s0
 ; CHECK-NEXT:    lea %s0, -32(, %s0)
-; CHECK-NEXT:    and %s0, %s0, (32)0
 ; CHECK-NEXT:    b.l.t (, %s10)
   %r = tail call i32 @llvm.ctlz.i32(i32 %p, i1 false)
   ret i32 %r