[llvm] [RISCV][MC] Add aliases for beq/bne with x0 as the first argument => beqz/bnez (PR #139086)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-mc

@llvm/pr-subscribers-backend-risc-v

Author: Alex Bradbury (asb)

<details>
<summary>Changes</summary>

I don't see a good reason not to use the alias for this form as well.
We have CompressPat for both ways round, so the difference isn't
meaningful from that perspective. Because we're mostly compiling with
RVC enabled, you only see the beq/bne forms with x0 as the first
operand in the case of the second operand being a register that isn't
compressible. I think this just makes assembly harder to read and sticks
out for no good reason in such cases.

---
Full diff: https://github.com/llvm/llvm-project/pull/139086.diff


2 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVInstrInfo.td (+4) 
- (modified) llvm/test/MC/RISCV/rvi-aliases-valid.s (+16-4) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.td b/llvm/lib/Target/RISCV/RISCVInstrInfo.td
index 4a4290483e94b..361233d20dc1c 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.td
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.td
@@ -965,8 +965,12 @@ def : InstAlias<"sgtu $rd, $rs, $rt", (SLTU GPR:$rd, GPR:$rt, GPR:$rs), 0>;
 
 def : InstAlias<"beqz $rs, $offset",
                 (BEQ GPR:$rs,      X0, bare_simm13_lsb0:$offset)>;
+def : InstAlias<"beqz $rs, $offset",
+                (BEQ      X0, GPR:$rs, bare_simm13_lsb0:$offset)>;
 def : InstAlias<"bnez $rs, $offset",
                 (BNE GPR:$rs,      X0, bare_simm13_lsb0:$offset)>;
+def : InstAlias<"bnez $rs, $offset",
+                (BNE      X0, GPR:$rs, bare_simm13_lsb0:$offset)>;
 def : InstAlias<"blez $rs, $offset",
                 (BGE      X0, GPR:$rs, bare_simm13_lsb0:$offset)>;
 def : InstAlias<"bgez $rs, $offset",
diff --git a/llvm/test/MC/RISCV/rvi-aliases-valid.s b/llvm/test/MC/RISCV/rvi-aliases-valid.s
index ef05d1295d44f..0082b12797da6 100644
--- a/llvm/test/MC/RISCV/rvi-aliases-valid.s
+++ b/llvm/test/MC/RISCV/rvi-aliases-valid.s
@@ -122,10 +122,22 @@ bgtu x17, x18, 28
 # CHECK-OBJ: bgeu s3, s2, 0x70
 bleu x18, x19, 32
 
+# Emit beqz/bnez alias even when operands are reversed to the canonical form.
+# CHECK-S-NOALIAS: beq zero, a0, 512
+# CHECK-S: beqz a0, 512
+# CHECK-OBJ-NOALIAS: beq zero, a0, 0x254
+# CHECK-OBJ: beqz a0, 0x254
+beq zero, x10, 512
+# CHECK-S-NOALIAS: bne zero, a1, 1024
+# CHECK-S: bnez a1, 1024
+# CHECK-OBJ-NOALIAS: bne zero, a1, 0x458
+# CHECK-OBJ: bnez a1, 0x458
+bne zero, x11, 1024
+
 # CHECK-S-NOALIAS: jal zero, 2044
 # CHECK-S: j 2044
-# CHECK-OBJ-NOALIAS: jal zero, 0x850
-# CHECK-OBJ: j 0x850
+# CHECK-OBJ-NOALIAS: jal zero, 0x858
+# CHECK-OBJ: j 0x858
 j 2044
 # CHECK-S-NOALIAS: jal zero, foo
 # CHECK-S: j foo
@@ -148,8 +160,8 @@ j a0
 j .
 # CHECK-S-NOALIAS: jal ra, 2040
 # CHECK-S: jal 2040
-# CHECK-OBJ-NOALIAS: jal ra, 0x85c
-# CHECK-OBJ: jal 0x85c
+# CHECK-OBJ-NOALIAS: jal ra, 0x864
+# CHECK-OBJ: jal 0x864
 jal 2040
 # CHECK-S-NOALIAS: jal ra, foo
 # CHECK-S: jal foo

``````````

</details>


https://github.com/llvm/llvm-project/pull/139086