[llvm] [Mips] Fix fast isel for i16 bswap. (PR #103398)

[Craig Topper via llvm-commits]

https://github.com/topperc created https://github.com/llvm/llvm-project/pull/103398

We need to mask the SRL result to 8 bits before ORing in the SLL. This is needed in case bits 23:16 of the input aren't zero. They will have been shifted into bits 15:8.

We don't need to AND the result with 0xffff. It's ok if the upper 16 bits of the register is garbage.

Fixes #103035.

>From 9cc6a7cb3ec511e8424034094cfb1aeed050b6a2 Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper at sifive.com>
Date: Tue, 13 Aug 2024 12:25:48 -0700
Subject: [PATCH] [Mips] Fix fast isel for i16 bswap.

We need to mask the SRL result to 8 bits before ORing in the
SLL. This is needed in case bits 23:16 of the input aren't zero.
They will have been shifted into bits 15:8.

We don't need to AND the result with 0xffff. It's ok if the
upper 16 bits of the register is garbage.

Fixes #103035.
---
 llvm/lib/Target/Mips/MipsFastISel.cpp      | 4 ++--
 llvm/test/CodeGen/Mips/Fast-ISel/bswap1.ll | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Target/Mips/MipsFastISel.cpp b/llvm/lib/Target/Mips/MipsFastISel.cpp
index 3485825b77627e..7d8278c8ca3cf5 100644
--- a/llvm/lib/Target/Mips/MipsFastISel.cpp
+++ b/llvm/lib/Target/Mips/MipsFastISel.cpp
@@ -1608,8 +1608,8 @@ bool MipsFastISel::fastLowerIntrinsicCall(const IntrinsicInst *II) {
         }
         emitInst(Mips::SLL, TempReg[0]).addReg(SrcReg).addImm(8);
         emitInst(Mips::SRL, TempReg[1]).addReg(SrcReg).addImm(8);
-        emitInst(Mips::OR, TempReg[2]).addReg(TempReg[0]).addReg(TempReg[1]);
-        emitInst(Mips::ANDi, DestReg).addReg(TempReg[2]).addImm(0xFFFF);
+        emitInst(Mips::ANDi, TempReg[2]).addReg(TempReg[1]).addImm(0xFF);
+        emitInst(Mips::OR, DestReg).addReg(TempReg[0]).addReg(TempReg[2]);
         updateValueMap(II, DestReg);
         return true;
       }
diff --git a/llvm/test/CodeGen/Mips/Fast-ISel/bswap1.ll b/llvm/test/CodeGen/Mips/Fast-ISel/bswap1.ll
index bd762a0e1d741f..ce664c78e86c2a 100644
--- a/llvm/test/CodeGen/Mips/Fast-ISel/bswap1.ll
+++ b/llvm/test/CodeGen/Mips/Fast-ISel/bswap1.ll
@@ -21,8 +21,8 @@ define void @b16() {
 
   ; 32R1:           sll   $[[TMP1:[0-9]+]], $[[A_VAL]], 8
   ; 32R1:           srl   $[[TMP2:[0-9]+]], $[[A_VAL]], 8
-  ; 32R1:           or    $[[TMP3:[0-9]+]], $[[TMP1]], $[[TMP2]]
-  ; 32R1:           andi  $[[TMP4:[0-9]+]], $[[TMP3]], 65535
+  ; 32R1:           andi  $[[TMP3:[0-9]+]], $[[TMP2]], 255
+  ; 32R1:           or    $[[RESULT:[0-9]+]], $[[TMP1]], $[[TMP3]]
 
   ; 32R2:           wsbh  $[[RESULT:[0-9]+]], $[[A_VAL]]