[RISCV] Add back handling of X > -1 to ISD::SETCC lowering.

[craig.topper]

Summary

There are cases where the -1 doesn't become visible until lowering
so the folding doesn't have a chance to run.

I think in these cases there is a missed DAGCombine for truncate (undef),
which I may fix separately, but RISC-V backend should protect itself.

Fixes #64503.

[asb]

LGTM. Worth considering for backporting to the 17.x branch too I think.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

if (OpVT.isScalarInteger()) {
 
       // If the RHS is a constant in the range [-2049, 0) or (0, 2046], we can
       // convert this to the equivalent of (set(u)ge X, C+1) by using
       // (xori (slti(u) X, C+1), 1). This avoids materializing a small constant
       // in a register.
       if (isa<ConstantSDNode>(RHS)) {
         int64_t Imm = cast<ConstantSDNode>(RHS)->getSExtValue();
         if (Imm != 0 && isInt<12>((uint64_t)Imm + 1)) {
-          // X > -1 should have been replaced with false.
-          assert((CCVal != ISD::SETUGT || Imm != -1) &&
-                 "Missing canonicalization");
+          // If this is an unsigned compare and the constant is -1, incrementing
+          // the constant would change behavior. The result should be false.
+          if (CCVal == ISD::SETUGT && Imm == -1)
+            return DAG.getConstant(0, DL, VT);
           // Using getSetCCSwappedOperands will convert SET(U)GT->SET(U)LT.
           CCVal = ISD::getSetCCSwappedOperands(CCVal);
           SDValue SetCC = DAG.getSetCC(
               DL, VT, LHS, DAG.getConstant(Imm + 1, DL, OpVT), CCVal);
           return DAG.getLogicalNOT(DL, SetCC, VT);
         }
       }
 

llvm/test/CodeGen/RISCV/pr64503.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 2
+; RUN: llc < %s -mtriple=riscv32 | FileCheck %s
+
+define i1 @f(i64 %LGV1) {
+; CHECK-LABEL: f:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    li a0, 1
+; CHECK-NEXT:    beqz a1, .LBB0_2
+; CHECK-NEXT:  # %bb.1:
+; CHECK-NEXT:    snez a0, a1
+; CHECK-NEXT:    xori a0, a0, 1
+; CHECK-NEXT:  .LBB0_2:
+; CHECK-NEXT:    ret
+  %B1 = xor i64 %LGV1, %LGV1
+  %B2 = srem i64 1, %B1
+  %B5 = lshr i64 1, %B2
+  %C4 = icmp ule i64 %LGV1, %B5
+  ret i1 %C4
+}
+
+define i64 @g(ptr %A, i64 %0) {
+; CHECK-LABEL: g:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    li a0, 1
+; CHECK-NEXT:    beqz a2, .LBB1_2
+; CHECK-NEXT:  # %bb.1:
+; CHECK-NEXT:    slti a0, a2, 1
+; CHECK-NEXT:  .LBB1_2:
+; CHECK-NEXT:    sb a0, 0(zero)
+; CHECK-NEXT:    ret
+  store i64 poison, ptr %A, align 4
+  %LGV1 = load i64, ptr %A, align 4
+  %B1 = ashr i64 1, %LGV1
+  %C = icmp sle i64 %0, %B1
+  store i1 %C, ptr null, align 1
+  ret i64 %LGV1
+}