[MachinePipeliner] Fix Phi refers to Phi in same stage in 1st epilogue

[jsji]

Summary

This is exposed by functional testing on PowerPC.
In some pipelined loops, Phi refer to phi did not get value defined by
the Phi, hence getting wrong value later.

As the comment mentioned, we should "use the value defined by the Phi,
unless we're generating the firstepilog and the Phi refers to a Phi
in a different stage.", so Phi refering to same stage Phi should use
the value defined by the Phi here.

[jsji]

Ping...

[hfinkel]

In D64035#1573615, @jsji wrote:

Ping...

LGTM

llvm/trunk/lib/CodeGen/MachinePipeliner.cpp

for (unsigned np = 0; np < NumPhis; ++np) {
           PhiOp2 = VRMap[PrevStage - np + 1][Def];
         // Use the loop value defined in the kernel.
         else if (static_cast<unsigned>(LoopValStage) > PrologStage + 1 &&
                  VRMap[PrevStage - StageDiffAdj - np].count(LoopVal))
           PhiOp2 = VRMap[PrevStage - StageDiffAdj - np][LoopVal];
         // Use the value defined by the Phi, unless we're generating the first
         // epilog and the Phi refers to a Phi in a different stage.
         else if (VRMap[PrevStage - np].count(Def) &&
-                 (!LoopDefIsPhi || PrevStage != LastStageNum))
+                 (!LoopDefIsPhi || (PrevStage != LastStageNum) || (LoopValStage == StageScheduled)))
           PhiOp2 = VRMap[PrevStage - np][Def];
       }
 
       // Check if we can reuse an existing Phi. This occurs when a Phi
       // references another Phi, and the other Phi is scheduled in an
       // earlier stage. We can try to reuse an existing Phi up until the last
       // stage of the current Phi.
       if (LoopDefIsPhi) {

llvm/trunk/test/CodeGen/PowerPC/sms-phi.ll

 ; REQUIRES: asserts
 ; RUN: llc < %s -mtriple=powerpc64le-unknown-linux-gnu -verify-machineinstrs\
 ; RUN:       -mcpu=pwr9 --ppc-enable-pipeliner -debug-only=pipeliner 2>&1 \
 ; RUN:       >/dev/null | FileCheck %s
 define dso_local void @sha512() #0 {
 ;CHECK: prolog:
 ;CHECK:        %16:g8rc = ADD8 %21:g8rc, %20:g8rc
 ;CHECK: epilog:
 ;CHECK:        %23:g8rc_and_g8rc_nox0 = PHI %5:g8rc_and_g8rc_nox0, %bb.3, %18:g8rc_and_g8rc_nox0, %bb.4
 ;CHECK-NEXT:   %24:g8rc = PHI %6:g8rc, %bb.3, %16:g8rc, %bb.4
-;CHECK-NEXT:   %25:g8rc = PHI %6:g8rc, %bb.3, %16:g8rc, %bb.4
+;CHECK-NEXT:   %25:g8rc = PHI %6:g8rc, %bb.3, %19:g8rc, %bb.4
   br label %1
 
 1:                                                ; preds = %1, %0
   %2 = phi i64 [ 0, %0 ], [ %12, %1 ]
   %3 = phi i64 [ undef, %0 ], [ %11, %1 ]
   %4 = phi i64 [ undef, %0 ], [ %3, %1 ]
   %5 = getelementptr inbounds [80 x i64], [80 x i64]* null, i64 0, i64 %2
   %6 = load i64, i64* %5, align 8