[EarlyIfConversion] Avoid producing selects with identical operands

[jroelofs]

Summary

This extends the early-ifcvt pass to avoid a few more cases where the resulting select instructions would have matching operands. Additionally, we now use TII to determine "sameness" of the operands so that as TII gets smarter, so too will ifcvt.

The attached test case was bugpoint-reduced down from CINT2000/252.eon in the test-suite. See: https://clang.godbolt.org/z/WvnrcrGEn

[xbolva00]

Please add a testcase

[jroelofs]

In D101508#2724583, @xbolva00 wrote:

Please add a testcase

oops. forgot to git-add. thanks for the reminder!

[thegameg]

llvm/test/CodeGen/AArch64/early-ifcvt-same-value.mir

2

mcpu needed?

[jroelofs]

llvm/test/CodeGen/AArch64/early-ifcvt-same-value.mir

2

Yes, otherwise the if-conversion doesn't fire.

[jroelofs]

llvm/test/CodeGen/AArch64/early-ifcvt-same-value.mir

2

erm, you're right. I must have needed that before I added -stress-early-ifcvt

[thegameg]

LGTM, thanks! In general, I like this direction more than the TII::optimizeSelect one.

[dmgreen]

llvm/lib/CodeGen/EarlyIfConversion.cpp

569

Does what you mentioned in D46278 apply here too? If the MachineInstr's have multiple defs, like a LDRpre or LDP, does it need to be careful which operand it is defined in and whether they match?

[jroelofs]

Handle instructions with multiple defs.

[dmgreen]

llvm/lib/CodeGen/EarlyIfConversion.cpp

560

Can this use MachineInstr::findRegisterDefOperandIdx?

[dmgreen]

Thanks. Sounds good to me too.

llvm/lib/CodeGen/EarlyIfConversion.cpp

bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB, bool Predicate) {
 
   if (isTriangle())
     ++NumTrianglesSeen;
   else
     ++NumDiamondsSeen;
   return true;
 }
 
+/// \return true iff the two registers are known to have the same value.
+static bool hasSameValue(const MachineRegisterInfo &MRI,
+                         const TargetInstrInfo *TII,
+                         Register TReg, Register FReg) {
+  if (TReg == FReg)
+    return true;
+
+  const MachineInstr *TDef = TReg.isVirtual() ? MRI.getVRegDef(TReg) : nullptr;
+  const MachineInstr *FDef = FReg.isVirtual() ? MRI.getVRegDef(FReg) : nullptr;
+  if (TDef && TDef == FDef)
+    return true;
+
+  if (TDef && FDef)
+    return TII->produceSameValue(*TDef, *FDef, &MRI);
+
+  return false;
+}
+
 /// replacePHIInstrs - Completely replace PHI instructions with selects.
 /// This is possible when the only Tail predecessors are the if-converted
 /// blocks.
 void SSAIfConv::replacePHIInstrs() {
   assert(Tail->pred_size() == 2 && "Cannot replace PHIs");
   MachineBasicBlock::iterator FirstTerm = Head->getFirstTerminator();
   assert(FirstTerm != Head->end() && "No terminators");
   DebugLoc HeadDL = FirstTerm->getDebugLoc();
 
   // Convert all PHIs to select instructions inserted before FirstTerm.
   for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
     PHIInfo &PI = PHIs[i];
     LLVM_DEBUG(dbgs() << "If-converting " << *PI.PHI);
     Register DstReg = PI.PHI->getOperand(0).getReg();
+    if (hasSameValue(*MRI, TII, PI.TReg, PI.FReg)) {
+      // We do not need the select instruction if both incoming values are
+      // equal, but we do need a COPY.
+      BuildMI(*Head, FirstTerm, HeadDL, TII->get(TargetOpcode::COPY), DstReg)
+        .addReg(PI.TReg);
+    } else {
       TII->insertSelect(*Head, FirstTerm, HeadDL, DstReg, Cond, PI.TReg, PI.FReg);
+    }
     LLVM_DEBUG(dbgs() << "          --> " << *std::prev(FirstTerm));
     PI.PHI->eraseFromParent();
     PI.PHI = nullptr;
   }
 }
 
 /// rewritePHIOperands - When there are additional Tail predecessors, insert
 /// select instructions in Head and rewrite PHI operands to use the selects.
 /// Keep the PHI instructions in Tail to handle the other predecessors.
 void SSAIfConv::rewritePHIOperands() {
   MachineBasicBlock::iterator FirstTerm = Head->getFirstTerminator();
   assert(FirstTerm != Head->end() && "No terminators");
   DebugLoc HeadDL = FirstTerm->getDebugLoc();
 
   // Convert all PHIs to select instructions inserted before FirstTerm.
   for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
     PHIInfo &PI = PHIs[i];
     unsigned DstReg = 0;
 
     LLVM_DEBUG(dbgs() << "If-converting " << *PI.PHI);
-    if (PI.TReg == PI.FReg) {
+    if (hasSameValue(*MRI, TII, PI.TReg, PI.FReg)) {
       // We do not need the select instruction if both incoming values are
       // equal.
       DstReg = PI.TReg;
     } else {
       Register PHIDst = PI.PHI->getOperand(0).getReg();
       DstReg = MRI->createVirtualRegister(MRI->getRegClass(PHIDst));
       TII->insertSelect(*Head, FirstTerm, HeadDL,
                          DstReg, Cond, PI.TReg, PI.FReg);

llvm/test/CodeGen/AArch64/early-ifcvt-same-value.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=aarch64-- -mcpu=apple-a7 -run-pass=early-ifcvt -stress-early-ifcvt -verify-machineinstrs %s -o - | FileCheck %s
+
+---
+name:            fmov0_diamond1
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: fpr32, preferred-register: '' }
+  - { id: 1, class: fpr32, preferred-register: '' }
+  - { id: 2, class: fpr32, preferred-register: '' }
+  - { id: 3, class: fpr32, preferred-register: '' }
+  - { id: 4, class: fpr32, preferred-register: '' }
+  - { id: 5, class: gpr32common, preferred-register: '' }
+  - { id: 6, class: gpr32, preferred-register: '' }
+  - { id: 7, class: fpr32, preferred-register: '' }
+  - { id: 8, class: fpr32, preferred-register: '' }
+liveins:
+  - { reg: '$s1', virtual-reg: '%4' }
+  - { reg: '$w0', virtual-reg: '%5' }
+body:             |
+  ; CHECK-LABEL: name: fmov0_diamond1
+  ; CHECK: bb.0.entry:
+  ; CHECK:   liveins: $s1, $w0
+  ; CHECK:   [[COPY:%[0-9]+]]:gpr32common = COPY $w0
+  ; CHECK:   [[COPY1:%[0-9]+]]:fpr32 = COPY $s1
+  ; CHECK:   [[SUBSWri:%[0-9]+]]:gpr32 = SUBSWri [[COPY]], 1, 0, implicit-def $nzcv
+  ; CHECK:   [[FMOVS0_:%[0-9]+]]:fpr32 = FMOVS0
+  ; CHECK:   [[FMOVS0_1:%[0-9]+]]:fpr32 = FMOVS0
+  ; CHECK:   [[COPY2:%[0-9]+]]:fpr32 = COPY [[FMOVS0_]]
+  ; CHECK:   $s0 = COPY [[COPY2]]
+  ; CHECK:   RET_ReallyLR implicit $s0
+  bb.0.entry:
+    successors: %bb.1, %bb.2
+    liveins: $s1, $w0
+
+    %5:gpr32common = COPY $w0
+    %4:fpr32 = COPY $s1
+    %6:gpr32 = SUBSWri %5, 1, 0, implicit-def $nzcv
+    Bcc 1, %bb.2, implicit $nzcv
+    B %bb.1
+
+  bb.1:
+    successors: %bb.3
+
+    %0:fpr32 = FMOVS0
+    B %bb.3
+
+  bb.2:
+    successors: %bb.3
+
+    %1:fpr32 = FMOVS0
+
+  bb.3:
+    %2:fpr32 = PHI %1, %bb.2, %0, %bb.1
+    $s0 = COPY %2
+    RET_ReallyLR implicit $s0
+
+...
+---
+name:            fmov0_diamond2
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: fpr32, preferred-register: '' }
+  - { id: 1, class: fpr32, preferred-register: '' }
+  - { id: 2, class: fpr32, preferred-register: '' }
+  - { id: 3, class: fpr32, preferred-register: '' }
+  - { id: 4, class: fpr32, preferred-register: '' }
+  - { id: 5, class: gpr32common, preferred-register: '' }
+  - { id: 6, class: gpr32, preferred-register: '' }
+  - { id: 7, class: fpr32, preferred-register: '' }
+  - { id: 8, class: fpr32, preferred-register: '' }
+liveins:
+  - { reg: '$s1', virtual-reg: '%4' }
+  - { reg: '$w0', virtual-reg: '%5' }
+body:             |
+  ; CHECK-LABEL: name: fmov0_diamond2
+  ; CHECK: bb.0.entry:
+  ; CHECK:   successors: %bb.4(0x80000000)
+  ; CHECK:   liveins: $s1, $w0
+  ; CHECK:   [[COPY:%[0-9]+]]:gpr32common = COPY $w0
+  ; CHECK:   [[COPY1:%[0-9]+]]:fpr32 = COPY $s1
+  ; CHECK:   [[SUBSWri:%[0-9]+]]:gpr32 = SUBSWri [[COPY]], 1, 0, implicit-def $nzcv
+  ; CHECK:   [[FMOVS0_:%[0-9]+]]:fpr32 = FMOVS0
+  ; CHECK:   [[FMOVS0_1:%[0-9]+]]:fpr32 = FMOVS0
+  ; CHECK:   B %bb.4
+  ; CHECK: bb.1:
+  ; CHECK:   successors: %bb.4(0x80000000)
+  ; CHECK:   [[DEF:%[0-9]+]]:fpr32 = IMPLICIT_DEF
+  ; CHECK:   B %bb.4
+  ; CHECK: bb.4:
+  ; CHECK:   [[PHI:%[0-9]+]]:fpr32 = PHI [[FMOVS0_]], %bb.0, [[DEF]], %bb.1
+  ; CHECK:   $s0 = COPY [[PHI]]
+  ; CHECK:   RET_ReallyLR implicit $s0
+  bb.0.entry:
+    successors: %bb.1, %bb.2
+    liveins: $s1, $w0
+
+    %5:gpr32common = COPY $w0
+    %4:fpr32 = COPY $s1
+    %6:gpr32 = SUBSWri %5, 1, 0, implicit-def $nzcv
+    Bcc 1, %bb.2, implicit $nzcv
+    B %bb.1
+
+  bb.4:
+    successors: %bb.3
+
+    ; Make sure we also handle the case when there are extra predecessors on
+    ; the tail block.
+    %3:fpr32 = IMPLICIT_DEF
+    B %bb.3
+
+  bb.1:
+    successors: %bb.3
+
+    %0:fpr32 = FMOVS0
+    B %bb.3
+
+  bb.2:
+    successors: %bb.3
+
+    %1:fpr32 = FMOVS0
+
+  bb.3:
+    %2:fpr32 = PHI %1, %bb.2, %0, %bb.1, %3, %bb.4
+    $s0 = COPY %2
+    RET_ReallyLR implicit $s0
+
+...