[PATCH] D116886: [M68k] Instruction selection to choose neg x when mul x -1 (Fix issue 48588)

[Douglas Chen via Phabricator via llvm-commits]

dougpuob updated this revision to Diff 400350.
dougpuob added a comment.

- Add a descriptive comment.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D116886/new/

https://reviews.llvm.org/D116886

Files:
  llvm/lib/Target/M68k/M68kISelDAGToDAG.cpp
  llvm/test/CodeGen/M68k/Arith/imul-neg.ll


Index: llvm/test/CodeGen/M68k/Arith/imul-neg.ll
===================================================================
--- llvm/test/CodeGen/M68k/Arith/imul-neg.ll
+++ llvm/test/CodeGen/M68k/Arith/imul-neg.ll
@@ -1,6 +1,28 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc < %s -mtriple=m68k-linux | FileCheck %s
 
+define i8 @mul255_8(i8 %A) {
+; CHECK-LABEL: mul255_8:
+; CHECK:         .cfi_startproc
+; CHECK-NEXT:  ; %bb.0:
+; CHECK-NEXT:    move.b	(7,%sp), %d0
+; CHECK-NEXT:    neg.b	%d0
+; CHECK-NEXT:    rts
+    %mul = mul i8 %A, 255
+    ret i8 %mul
+}
+
+define i16 @mul65535_16(i16 %A) {
+; CHECK-LABEL: mul65535_16:
+; CHECK:         .cfi_startproc
+; CHECK-NEXT:  ; %bb.0:
+; CHECK-NEXT:    move.w	(6,%sp), %d0
+; CHECK-NEXT:    neg.w	%d0
+; CHECK-NEXT:    rts
+    %mul = mul i16 %A, 65535
+    ret i16 %mul
+}
+
 define i32 @mul4294967295_32(i32 %A) {
 ; CHECK-LABEL: mul4294967295_32:
 ; CHECK:         .cfi_startproc
Index: llvm/lib/Target/M68k/M68kISelDAGToDAG.cpp
===================================================================
--- llvm/lib/Target/M68k/M68kISelDAGToDAG.cpp
+++ llvm/lib/Target/M68k/M68kISelDAGToDAG.cpp
@@ -322,13 +322,17 @@
     default:
       return true;
     case M68kISD::SUB:
-    case ISD::SUB: {
-      SDNode *Dividend = U->getOperand(0).getNode();
-      if (ConstantSDNode *CSDN = dyn_cast<ConstantSDNode>(Dividend))
-        if (const uint64_t *RawData = CSDN->getAPIntValue().getRawData())
-          if (0 == *RawData)
-            return false;
-    }
+    case ISD::SUB:
+      // Prefer NEG instruction when zero subtracts a value.
+      // e.g.
+      //   move.l	#0, %d0
+      //   sub.l	(4,%sp), %d0
+      // vs.
+      //   move.l	(4,%sp), %d0
+      //   neg.l	%d0
+      if (llvm::isNullConstant(U->getOperand(0)))
+        return false;
+      break;
     }
   }
 


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