[PATCH] D146820: [AArch64][PeepholeOpt]Optimize ALU + compare to flag-setting ALU

[Mingming Liu via Phabricator via llvm-commits]

mingmingl created this revision.
Herald added subscribers: hiraditya, kristof.beyls.
Herald added a project: All.
mingmingl requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D146820

Files:
  llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
  llvm/test/CodeGen/AArch64/arm64-csel.ll


Index: llvm/test/CodeGen/AArch64/arm64-csel.ll
===================================================================
--- llvm/test/CodeGen/AArch64/arm64-csel.ll
+++ llvm/test/CodeGen/AArch64/arm64-csel.ll
@@ -79,8 +79,7 @@
 define i32 at foo6(i32 %a, i32 %b) nounwind ssp {
 ; CHECK-LABEL: foo6:
 ; CHECK:       // %bb.0: // %common.ret
-; CHECK-NEXT:    sub w8, w0, w1
-; CHECK-NEXT:    cmp w8, #0
+; CHECK-NEXT:    subs w8, w0, w1
 ; CHECK-NEXT:    csinc w0, w8, wzr, le
 ; CHECK-NEXT:    ret
   %sub = sub nsw i32 %a, %b
Index: llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
===================================================================
--- llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
+++ llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -1692,17 +1692,23 @@
 ///        MI and CmpInstr
 ///        or if MI opcode is not the S form there must be neither defs of flags
 ///        nor uses of flags between MI and CmpInstr.
-/// - and  C/V flags are not used after CmpInstr
+/// - and  C flag is not used after CmpInstr
 static bool canInstrSubstituteCmpInstr(MachineInstr &MI, MachineInstr &CmpInstr,
                                        const TargetRegisterInfo &TRI) {
+  // NOTE this assertion guarantees that MI.getOpcode() is the S-form of add or
+  // subtraction.
   assert(sForm(MI) != AArch64::INSTRUCTION_LIST_END);
 
   const unsigned CmpOpcode = CmpInstr.getOpcode();
   if (!isADDSRegImm(CmpOpcode) && !isSUBSRegImm(CmpOpcode))
     return false;
 
+  assert((CmpInstr.getOperand(2).isImm() &&
+          CmpInstr.getOperand(2).getImm() == 0) &&
+         "Caller guarantees that CmpInstr compares with constant 0");
+
   std::optional<UsedNZCV> NZVCUsed = examineCFlagsUse(MI, CmpInstr, TRI);
-  if (!NZVCUsed || NZVCUsed->C || NZVCUsed->V)
+  if (!NZVCUsed || NZVCUsed->C)
     return false;
 
   AccessKind AccessToCheck = AK_Write;


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