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[PHIElimination] Reuse existing COPY in predecessor basic block
Simplifies the CFG later on, added a regression test.
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‎llvm/lib/CodeGen/PHIElimination.cpp

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@@ -587,6 +587,15 @@ void PHIEliminationImpl::LowerPHINode(MachineBasicBlock &MBB,
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MachineBasicBlock::iterator InsertPos =
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findPHICopyInsertPoint(&opBlock, &MBB, SrcReg);
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// Reuse an existing copy in the block if possible.
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if (MachineInstr *DefMI = MRI->getUniqueVRegDef(SrcReg)) {
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if (DefMI->isCopy() && DefMI->getParent() == &opBlock &&
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MRI->use_empty(SrcReg)) {
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DefMI->getOperand(0).setReg(IncomingReg);
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continue;
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}
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}
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// Insert the copy.
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MachineInstr *NewSrcInstr = nullptr;
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if (!reusedIncoming && IncomingReg) {

‎llvm/test/CodeGen/AArch64/PHIElimination-reuse-copy.mir

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# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
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# RUN: llc -run-pass=phi-node-elimination -mtriple=aarch64-linux-gnu -o - %s | FileCheck %s
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# Verify that the original COPY in bb.1 is reappropriated as the PHI source in bb.2,
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# instead of creating a new COPY with the same source register.
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---
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name: test
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tracksRegLiveness: true
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body: |
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; CHECK-LABEL: name: test
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; CHECK: bb.0:
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; CHECK-NEXT: successors: %bb.2(0x40000000), %bb.1(0x40000000)
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; CHECK-NEXT: liveins: $nzcv, $wzr
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; CHECK-NEXT: {{ $}}
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; CHECK-NEXT: [[DEF:%[0-9]+]]:gpr32 = IMPLICIT_DEF
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; CHECK-NEXT: Bcc 8, %bb.2, implicit $nzcv
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; CHECK-NEXT: {{ $}}
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; CHECK-NEXT: bb.1:
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; CHECK-NEXT: successors: %bb.2(0x80000000)
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; CHECK-NEXT: {{ $}}
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; CHECK-NEXT: [[DEF:%[0-9]+]]:gpr32 = COPY $wzr
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; CHECK-NEXT: {{ $}}
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; CHECK-NEXT: bb.2:
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; CHECK-NEXT: %y:gpr32 = COPY [[DEF]]
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; CHECK-NEXT: $wzr = COPY %y
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bb.0:
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liveins: $nzcv, $wzr
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Bcc 8, %bb.2, implicit $nzcv
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bb.1:
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%x:gpr32 = COPY $wzr
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bb.2:
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%y:gpr32 = PHI %x:gpr32, %bb.1, undef %undef:gpr32, %bb.0
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$wzr = COPY %y:gpr32
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...

‎llvm/test/CodeGen/AArch64/block-layout-regression.mir

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# RUN: llc -mtriple=aarch64-linux-gnu -verify-machineinstrs -o - %s -start-before=phi-node-elimination -stop-after=block-placement | FileCheck %s
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# TODO: Reduce test case.
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# CHECK-NOT: CBNZX
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---
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name: test
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tracksRegLiveness: true
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body: |
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bb.0:
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liveins: $x0, $w1, $x2, $x3
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%42:gpr64all = COPY killed $x0
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%41:gpr32all = COPY killed $w1
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%120:gpr64common = COPY killed $x2
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%121:gpr64common = COPY killed $x3
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B %bb.20
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bb.20:
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successors: %bb.21(0x30000000), %bb.22(0x50000000); %bb.21(37.50%), %bb.22(62.50%)
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%43:gpr64common = PHI %42:gpr64all, %bb.0, %55:gpr64common, %bb.28
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%44:gpr32all = PHI %41:gpr32all, %bb.0, %57:gpr32all, %bb.28
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%144:gpr64 = LDRXui %121:gpr64common, 0 :: (load (s64))
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%145:gpr64 = LDRXui %43:gpr64common, 0 :: (load (s64))
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%146:gpr64 = EORXrr killed %145:gpr64, killed %144:gpr64
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STRXui killed %43:gpr64common, %121:gpr64common, 0 :: (store (s64))
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STRXui %146:gpr64, %121:gpr64common, 1 :: (store (s64))
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%147:gpr64 = LDRXui %120:gpr64common, 1 :: (load (s64))
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CBNZX %147:gpr64, %bb.22
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bb.21:
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successors: %bb.26(0x80000000); %bb.26(100.00%)
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%45:gpr64all = COPY killed %146:gpr64
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B %bb.26
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bb.22:
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successors: %bb.23(0x80000000); %bb.23(100.00%)
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%46:gpr64all = COPY killed %147:gpr64
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%148:gpr64 = LDRXui %120:gpr64common, 0 :: (load (s64))
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%47:gpr64all = COPY killed %148:gpr64
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bb.23:
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successors: %bb.25(0x04000000), %bb.24(0x7c000000); %bb.25(3.12%), %bb.24(96.88%)
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%48:gpr32common = PHI %44:gpr32all, %bb.22, %51:gpr32all, %bb.24
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%49:gpr64common = PHI %46:gpr64all, %bb.22, %52:gpr64all, %bb.24
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%50:gpr64 = PHI %47:gpr64all, %bb.22, %49:gpr64common, %bb.24
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%149:gpr32 = LDRWui %49:gpr64common, 2 :: (load (s32))
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%150:gpr32common = nsw ADDWri %48:gpr32common, 1, 0
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dead $wzr = SUBSWrr killed %149:gpr32, killed %150:gpr32common, implicit-def $nzcv
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Bcc 12, %bb.25, implicit killed $nzcv
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B %bb.24
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bb.24:
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successors: %bb.25(0x04000000), %bb.23(0x7c000000); %bb.25(3.12%), %bb.23(96.88%)
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%152:gpr32 = LDRWui %49:gpr64common, 3 :: (load (s32))
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dead $wzr = SUBSWrr %48:gpr32common, %152:gpr32, implicit-def $nzcv
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%154:gpr32 = CSELWr killed %48:gpr32common, killed %152:gpr32, 12, implicit killed $nzcv
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%51:gpr32all = COPY killed %154:gpr32
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%155:gpr64 = LDRXui %49:gpr64common, 0 :: (load (s64))
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%156:gpr64 = EORXrr %155:gpr64, %50:gpr64
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%52:gpr64all = COPY %156:gpr64
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STRXui %49:gpr64common, %120:gpr64common, 0 :: (store (s64))
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STRXui killed %156:gpr64, %120:gpr64common, 1 :: (store (s64))
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dead $xzr = SUBSXrr killed %155:gpr64, killed %50:gpr64, implicit-def $nzcv
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Bcc 1, %bb.23, implicit killed $nzcv
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B %bb.25
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bb.25:
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successors: %bb.26(0x80000000); %bb.26(100.00%)
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%53:gpr32all = PHI %48:gpr32common, %bb.23, %51:gpr32all, %bb.24
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%158:gpr64 = LDRXui %121:gpr64common, 1 :: (load (s64))
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%54:gpr64all = COPY killed %158:gpr64
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bb.26:
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successors: %bb.29(0x04000000), %bb.27(0x7c000000); %bb.29(3.12%), %bb.27(96.88%)
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%55:gpr64common = PHI %45:gpr64all, %bb.21, %54:gpr64all, %bb.25
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%56:gpr32common = PHI %44:gpr32all, %bb.21, %53:gpr32all, %bb.25
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CBZX %55:gpr64common, %bb.29
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B %bb.27
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bb.27:
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successors: %bb.29(0x04000000), %bb.28(0x7c000000); %bb.29(3.12%), %bb.28(96.88%)
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%159:gpr32 = LDRWui %55:gpr64common, 2 :: (load (s32))
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%160:gpr32common = nsw ADDWri %56:gpr32common, 1, 0
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dead $wzr = SUBSWrr killed %159:gpr32, killed %160:gpr32common, implicit-def $nzcv
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Bcc 12, %bb.29, implicit killed $nzcv
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B %bb.28
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bb.28:
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successors: %bb.20(0x80000000); %bb.20(100.00%)
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%162:gpr32 = LDRWui %55:gpr64common, 3 :: (load (s32))
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dead $wzr = SUBSWrr %56:gpr32common, %162:gpr32, implicit-def $nzcv
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%164:gpr32 = CSELWr killed %56:gpr32common, killed %162:gpr32, 12, implicit killed $nzcv
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%57:gpr32all = COPY killed %164:gpr32
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B %bb.20
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bb.29:
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RET_ReallyLR
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...

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