[LoopPeeling] Fix condition for phi-eliminating peeling

[mkazantsev]

Summary

When peeling loops basing on phis becoming invariants, we make a wrong loop size check.
UP.Threshold should be compared against the total numbers of instructions after the transformation,
which is equal to 2 * LoopSize in case of peeling one iteration.
We should also check that the maximum allowed number of peeled iterations is not zero.

[mkazantsev]

Need to add a negative test when the peeling doesn't happen due to UnrollPeelMaxCount = 0.

[mkuper]

LGTM

lib/Transforms/Utils/LoopUnrollPeel.cpp

void llvm::computePeelCount(Loop *L, unsigned LoopSize,
   // Only try to peel innermost loops.
   if (!L->empty())
     return;
 
   // Try to find a Phi node that has the same loop invariant as an input from
   // its only back edge. If there is such Phi, peeling 1 iteration from the
   // loop is profitable, because starting from 2nd iteration we will have an
   // invariant instead of this Phi.
-  if (LoopSize <= UP.Threshold) {
+  // First, check that we can peel at least one iteration.
+  if (2 * LoopSize <= UP.Threshold && UnrollPeelMaxCount > 0) {
     BasicBlock *BackEdge = L->getLoopLatch();
     assert(BackEdge && "Loop is not in simplified form?");
     BasicBlock *Header = L->getHeader();
     // Iterate over Phis to find one with invariant input on back edge.
     bool FoundCandidate = false;
     PHINode *Phi;
     for (auto BI = Header->begin(); isa<PHINode>(&*BI); ++BI) {
       Phi = cast<PHINode>(&*BI);

test/Transforms/LoopUnroll/peel-loop-not-forced.ll

-; RUN: opt < %s -S -loop-unroll -unroll-threshold=4 | FileCheck %s
+; RUN: opt < %s -S -loop-unroll -unroll-threshold=8 | FileCheck %s
 
 define i32 @invariant_backedge_1(i32 %a, i32 %b) {
 ; CHECK-LABEL: @invariant_backedge_1
 ; CHECK-NOT:     %plus = phi
 ; CHECK:       loop.peel:
 ; CHECK:       loop:
 ; CHECK:         %i = phi
 ; CHECK:         %sum = phi