[PowerPC] Remove self-copies in pre-emit peephole

[nemanjai]

Summary

There are occasionally instances where AADB rewrites registers in such a way that a reg-reg copy becomes a self-copy. Such an instruction is obviously redundant and can be removed. This patch does precisely that.

Note that this will not remove various nop's that we insert (which are themselves just self-copies). The reason those are left alone is that all of them have their own opcodes (that just encode to a self-copy).

What prompted this patch is the fact that these self-copies sometimes end up using registers that make the instruction a priority-setting nop, thereby having a significant effect on performance.

[hfinkel]

LGTM

lib/Target/PowerPC/PPCPreEmitPeephole.cpp

69

Please add a comment here stating that these can come out of the AADB.

[jsji]

It would be great if we can avoid generating self-copies in AADB and others (no sure whether it is easy or possible there). But yes, it is always great to put this sentinel guard here. Thanks!

lib/Target/PowerPC/PPCPreEmitPeephole.cpp

72

How about PPC::MCRF, PPC::CROR,PPC::EVOR? QPX copies if we still support QPX?

80

Maybe we should continue (skip the following check) once we identified that this is to be erased?

[nemanjai]

If the reviewers agree with my proposals for addressing the comments, I'll address them on the commit.

lib/Target/PowerPC/PPCPreEmitPeephole.cpp

69

Will add it on the commit. Thanks.

72

I didn't add the SPE and QPX ones because I can't test them. Of course, this should be a safe thing to do no matter what, but I am reluctant to add code I can't functionally test.
@hfinkel Do you think I should go ahead and add the following:

PPC::MCRF
PPC::QVFMR
PPC::QVFMRs
PPC::QVFMRb
PPC::CROR
PPC::EVOR

80

That's a good point. I'll update the patch to add a continue after either of the updates to InstrsToErase since we don't need to try to convert it to an R+I instruction if we know we're deleting it.

[jsji]

SGTM.

[hfinkel]

lib/Target/PowerPC/PPCPreEmitPeephole.cpp

72

Seems safe enough. It has the benefit of not being an "arbitrary" subset. Downside is that it's harder to test. I'm fine with adding them in this case.

lib/Target/PowerPC/PPCPreEmitPeephole.cpp

 using namespace llvm;
 
 #define DEBUG_TYPE "ppc-pre-emit-peephole"
 
 STATISTIC(NumRRConvertedInPreEmit,
           "Number of r+r instructions converted to r+i in pre-emit peephole");
 STATISTIC(NumRemovedInPreEmit,
           "Number of instructions deleted in pre-emit peephole");
+STATISTIC(NumberOfSelfCopies,
+          "Number of self copy instructions eliminated");
 
 static cl::opt<bool>
 RunPreEmitPeephole("ppc-late-peephole", cl::Hidden, cl::init(true),
                    cl::desc("Run pre-emit peephole optimizations."));
 
 namespace {
   class PPCPreEmitPeephole : public MachineFunctionPass {
   public:
public:
     bool runOnMachineFunction(MachineFunction &MF) override {
       if (skipFunction(MF.getFunction()) || !RunPreEmitPeephole)
         return false;
       bool Changed = false;
       const PPCInstrInfo *TII = MF.getSubtarget<PPCSubtarget>().getInstrInfo();
       SmallVector<MachineInstr *, 4> InstrsToErase;
       for (MachineBasicBlock &MBB : MF) {
         for (MachineInstr &MI : MBB) {
+          unsigned Opc = MI.getOpcode();
+          // Detect self copies.
+          if (Opc == PPC::OR || Opc == PPC::OR8 || Opc == PPC::FMR ||
+              Opc == PPC::VOR || Opc == PPC::XXLOR || Opc == PPC::XXLORf ||
+              Opc == PPC::XSCPSGNDP) {
+            const MCInstrDesc &MCID = TII->get(Opc);
+            if (MCID.getNumOperands() == 3 &&
+                MI.getOperand(0).getReg() == MI.getOperand(1).getReg() &&
+                MI.getOperand(0).getReg() == MI.getOperand(2).getReg()) {
+              NumberOfSelfCopies++;
+              LLVM_DEBUG(dbgs() << "Deleting self-copy instruction: ");
+              LLVM_DEBUG(MI.dump());
+              InstrsToErase.push_back(&MI);
+            }
+            else if (MCID.getNumOperands() == 2 &&
+                     MI.getOperand(0).getReg() == MI.getOperand(1).getReg()) {
+              NumberOfSelfCopies++;
+              LLVM_DEBUG(dbgs() << "Deleting self-copy instruction: ");
+              LLVM_DEBUG(MI.dump());
+              InstrsToErase.push_back(&MI);
+            }
+          }
           MachineInstr *DefMIToErase = nullptr;
           if (TII->convertToImmediateForm(MI, &DefMIToErase)) {
             Changed = true;
             NumRRConvertedInPreEmit++;
             LLVM_DEBUG(dbgs() << "Converted instruction to imm form: ");
             LLVM_DEBUG(MI.dump());
             if (DefMIToErase) {
               InstrsToErase.push_back(DefMIToErase);

test/CodeGen/PowerPC/remove-self-copies.mir

+# RUN: llc -start-before ppc-pre-emit-peephole \
+# RUN:   -verify-machineinstrs -ppc-asm-full-reg-names %s -o - | FileCheck %s
+--- |
+  ; ModuleID = 't.ll'
+  source_filename = "t.ll"
+  target datalayout = "e-m:e-i64:64-n32:64"
+  
+  define signext i32 @test(i32 signext %a, i32 signext %b, i32 signext %c) {
+  entry:
+    %cmp = icmp slt i32 %a, %b
+    br i1 %cmp, label %return, label %if.end
+  
+  if.end:                                           ; preds = %entry
+    %cmp1 = icmp slt i32 %b, %a
+    br i1 %cmp1, label %return, label %if.end3
+  
+  if.end3:                                          ; preds = %if.end
+    %cmp4 = icmp eq i32 %a, %c
+    br i1 %cmp4, label %if.then5, label %if.end6
+  
+  if.then5:                                         ; preds = %if.end3
+    %add = shl nsw i32 %a, 1
+    br label %return
+  
+  if.end6:                                          ; preds = %if.end3
+    %cmp7 = icmp sgt i32 %c, %b
+    %add11 = add i32 %c, %b
+    %add12 = select i1 %cmp7, i32 %a, i32 0
+    %spec.select = add i32 %add11, %add12
+    ret i32 %spec.select
+  
+  return:                                           ; preds = %if.then5, %if.end, %entry
+    %retval.0 = phi i32 [ %add, %if.then5 ], [ %c, %entry ], [ %b, %if.end ]
+    ret i32 %retval.0
+  }
+
+...
+---
+name:            test
+alignment:       4
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+registers:       
+liveins:         
+  - { reg: '$x3', virtual-reg: '' }
+  - { reg: '$x4', virtual-reg: '' }
+  - { reg: '$x5', virtual-reg: '' }
+frameInfo:       
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    0
+  adjustsStack:    false
+  hasCalls:        false
+  stackProtector:  ''
+  maxCallFrameSize: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      
+stack:           
+constants:       
+body:             |
+  bb.0.entry:
+    successors: %bb.4(0x40000000), %bb.1(0x40000000)
+    liveins: $x3, $x4, $x5
+  
+    renamable $cr0 = CMPW renamable $r3, renamable $r4
+    BCC 12, renamable $cr0, %bb.4
+  
+  bb.1.if.end:
+    successors: %bb.6(0x40000000), %bb.2(0x40000000)
+    liveins: $cr0, $x3, $x4, $x5
+  
+    BCC 36, killed renamable $cr0, %bb.2
+  
+  bb.6:
+    liveins: $x4
+  
+    ; CHECK: mr r5, r4
+    ; CHECK-NOT: mr r5, r5
+    ; CHECK: extsw r3, r5
+    $r5 = OR killed $r4, $r4, implicit $x4, implicit-def $x5
+    $r5 = OR $r5, $r5, implicit-def $x5
+    renamable $x3 = EXTSW_32_64 killed renamable $r5, implicit $x5
+    BLR8 implicit $lr8, implicit $rm, implicit killed $x3
+  
+  bb.2.if.end3:
+    successors: %bb.3(0x40000000), %bb.5(0x40000000)
+    liveins: $x3, $x4, $x5
+  
+    renamable $cr0 = CMPLW renamable $r3, renamable $r5
+    BCC 68, killed renamable $cr0, %bb.5
+  
+  bb.3.if.then5:
+    successors: %bb.4(0x80000000)
+    liveins: $x3
+  
+    renamable $r5 = RLWINM killed renamable $r3, 1, 0, 30, implicit $x3, implicit-def $x5
+  
+  bb.4.return:
+    liveins: $x5
+  
+    renamable $x3 = EXTSW_32_64 killed renamable $r5, implicit $x5
+    BLR8 implicit $lr8, implicit $rm, implicit killed $x3
+  
+  bb.5.if.end6:
+    liveins: $x3, $x4, $x5
+  
+    renamable $cr0 = CMPW renamable $r5, renamable $r4
+    renamable $r6 = LI 0
+    renamable $r4 = ADD4 killed renamable $r5, killed renamable $r4, implicit $x4, implicit $x5
+    renamable $r3 = ISEL killed renamable $r3, killed renamable $r6, killed renamable $cr0gt, implicit $cr0, implicit $x3
+    renamable $r3 = ADD4 killed renamable $r4, killed renamable $r3
+    renamable $x3 = EXTSW_32_64 killed renamable $r3
+    BLR8 implicit $lr8, implicit $rm, implicit killed $x3
+
+...