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

[Alex Bradbury via llvm-commits]

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

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.

>From 8825ad49adc1c1ec278378820809727e4138aa21 Mon Sep 17 00:00:00 2001
From: Alex Bradbury <asb at igalia.com>
Date: Thu, 8 May 2025 14:59:36 +0100
Subject: [PATCH 1/2] [RISCV][MC][test] Pre-commit test case for beqz/bnez
 alias addition

---
 llvm/test/MC/RISCV/rvi-aliases-valid.s | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/llvm/test/MC/RISCV/rvi-aliases-valid.s b/llvm/test/MC/RISCV/rvi-aliases-valid.s
index ef05d1295d44f..b58e903522f83 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
 
+# beqz/bnez where operands are reversed to the canonical form.
+# CHECK-S-NOALIAS: beq zero, a0, 512
+# CHECK-S: beq zero, a0, 512
+# CHECK-OBJ-NOALIAS: beq zero, a0, 0x254
+# CHECK-OBJ: beq zero, a0, 0x254
+beq zero, x10, 512
+# CHECK-S-NOALIAS: bne zero, a1, 1024
+# CHECK-S: bne zero, a1, 1024
+# CHECK-OBJ-NOALIAS: bne zero, a1, 0x458
+# CHECK-OBJ: bne zero, 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

>From 07f78eac87cef02500e6c147f17be6fd994de287 Mon Sep 17 00:00:00 2001
From: Alex Bradbury <asb at igalia.com>
Date: Thu, 8 May 2025 15:06:09 +0100
Subject: [PATCH 2/2] [RISCV][MC] Add aliases for beqz/bnez with x0 as the
 first argument

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.
---
 llvm/lib/Target/RISCV/RISCVInstrInfo.td |  4 ++++
 llvm/test/MC/RISCV/rvi-aliases-valid.s  | 10 +++++-----
 2 files changed, 9 insertions(+), 5 deletions(-)

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 b58e903522f83..0082b12797da6 100644
--- a/llvm/test/MC/RISCV/rvi-aliases-valid.s
+++ b/llvm/test/MC/RISCV/rvi-aliases-valid.s
@@ -122,16 +122,16 @@ bgtu x17, x18, 28
 # CHECK-OBJ: bgeu s3, s2, 0x70
 bleu x18, x19, 32
 
-# beqz/bnez where operands are reversed to the canonical form.
+# Emit beqz/bnez alias even when operands are reversed to the canonical form.
 # CHECK-S-NOALIAS: beq zero, a0, 512
-# CHECK-S: beq zero, a0, 512
+# CHECK-S: beqz a0, 512
 # CHECK-OBJ-NOALIAS: beq zero, a0, 0x254
-# CHECK-OBJ: beq zero, a0, 0x254
+# CHECK-OBJ: beqz a0, 0x254
 beq zero, x10, 512
 # CHECK-S-NOALIAS: bne zero, a1, 1024
-# CHECK-S: bne zero, a1, 1024
+# CHECK-S: bnez a1, 1024
 # CHECK-OBJ-NOALIAS: bne zero, a1, 0x458
-# CHECK-OBJ: bne zero, a1, 0x458
+# CHECK-OBJ: bnez a1, 0x458
 bne zero, x11, 1024
 
 # CHECK-S-NOALIAS: jal zero, 2044