[llvm] [RISCV] Canonicalize beq/bne with x0 as first arg to beqz/bnez (PR #141781)

[Alex Bradbury via llvm-commits]

https://github.com/asb updated https://github.com/llvm/llvm-project/pull/141781

>From 78b1681b565a8e795314b6167fac27a5b11b7218 Mon Sep 17 00:00:00 2001
From: Alex Bradbury <asb at igalia.com>
Date: Wed, 28 May 2025 15:41:24 +0100
Subject: [PATCH] [RISCV] Canonicalize beq/bne with x0 as first arg to
 beqz/bnez

This covers similar ground as #139086. Although I think it makes sense
to land both, the practical motivation for #139086 is significantly
reduced here.

This canonicalisation makes no difference to compressibility (we have
compress patterns for the operands in either order), but it does mean
that easier to read assembly is printed, as we don't have aliases
defined for beq/bne with x0 in either position.
---
 llvm/lib/Target/RISCV/RISCVInstrInfo.cpp      |  9 +++++
 .../machine-copyprop-simplifyinstruction.mir  | 36 +++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
index 818dedac08dde..45fa26d1890db 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
@@ -4191,6 +4191,15 @@ bool RISCVInstrInfo::simplifyInstruction(MachineInstr &MI) const {
       return true;
     }
     break;
+  case RISCV::BEQ:
+  case RISCV::BNE:
+    // b{eq,ne} zero, rs, imm => b{eq,ne} 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});
+    }
+    break;
   }
   return false;
 }
diff --git a/llvm/test/CodeGen/RISCV/machine-copyprop-simplifyinstruction.mir b/llvm/test/CodeGen/RISCV/machine-copyprop-simplifyinstruction.mir
index 15a6d53f343c1..6d47e7084df25 100644
--- a/llvm/test/CodeGen/RISCV/machine-copyprop-simplifyinstruction.mir
+++ b/llvm/test/CodeGen/RISCV/machine-copyprop-simplifyinstruction.mir
@@ -742,3 +742,39 @@ body: |
     renamable $x10 = MAXU renamable $x11, renamable $x11
     PseudoRET implicit $x10
 ...
+---
+name: beq
+body: |
+  ; CHECK-LABEL: name: beq
+  ; 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
+    BEQ $x0, renamable $x11, %bb.1
+  bb.1:
+    PseudoRET
+...
+---
+name: bne
+body: |
+  ; CHECK-LABEL: name: bne
+  ; 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
+    BNE $x0, renamable $x11, %bb.1
+  bb.1:
+    PseudoRET
+...