[RISCV] Add bltu/bgeu zero => bnez/beqz canonicalisation to RISCVInstrInfo::simplifyInstruction (#141775)

We can end up with bltu/bgeu with the zero register as the 0th operand
after tail duplication and then machine copy propagation. Canonicalising
in simplifyInstruction (called from MachineCopyPropagation) both
produces easier to read assembly output, and means it's possible that
the compressed c.beqz/c.bnez forms are produced.

Author: asb

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -4189,6 +4189,24 @@ bool RISCVInstrInfo::simplifyInstruction(MachineInstr &MI) const {
       return true;
     }
     break;
+  case RISCV::BLTU:
+    // bltu zero, rs, imm => bne rs, zero, imm
+    if (MI.getOperand(0).getReg() == RISCV::X0) {
+      MachineOperand MO0 = MI.getOperand(0);
+      MI.removeOperand(0);
+      MI.insert(MI.operands_begin() + 1, {MO0});
+      MI.setDesc(get(RISCV::BNE));
+    }
+    break;
+  case RISCV::BGEU:
+    // bgeu zero, rs, imm => beq rs, zero, imm
+    if (MI.getOperand(0).getReg() == RISCV::X0) {
+      MachineOperand MO0 = MI.getOperand(0);
+      MI.removeOperand(0);
+      MI.insert(MI.operands_begin() + 1, {MO0});
+      MI.setDesc(get(RISCV::BEQ));
+    }
+    break;
   }
   return false;
 }

llvm/test/CodeGen/RISCV/machine-copyprop-simplifyinstruction.mir

@@ -742,3 +742,39 @@ body: |
     renamable $x10 = MAXU renamable $x11, renamable $x11
     PseudoRET implicit $x10
 ...
+---
+name: bltu
+body: |
+  ; CHECK-LABEL: name: bltu
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   renamable $x11 = COPY $x12
+  ; CHECK-NEXT:   BNE $x12, $x0, %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   PseudoRET
+  bb.0:
+    renamable $x11 = COPY $x12
+    BLTU $x0, renamable $x11, %bb.1
+  bb.1:
+    PseudoRET
+...
+---
+name: bgeu
+body: |
+  ; CHECK-LABEL: name: bgeu
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   renamable $x11 = COPY $x12
+  ; CHECK-NEXT:   BEQ $x12, $x0, %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   PseudoRET
+  bb.0:
+    renamable $x11 = COPY $x12
+    BGEU $x0, renamable $x11, %bb.1
+  bb.1:
+    PseudoRET
+...