[LoopUnroll] Use LoopSize+1 as threshold, to allow unrolling loops matching LoopSize.

[fhahn]

Summary

We use < UP.Threshold later on, so we should use LoopSize + 1, to
allow unrolling if the result won't exceed to loop size.

Fixes PR43305.

[dmgreen]

So we unroll the loop if the sizes are equal too? Sounds sensible to me.

llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp

1036

Can you update this comment with why it's +1.

[fhahn]

Adjust comment and test case.

[fhahn]

llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp

1036

done, thanks!

[dmgreen]

LGTM

llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp

static LoopUnrollResult tryToUnrollLoop(
   LLVM_DEBUG(dbgs() << "  Loop Size = " << LoopSize << "\n");
   if (NotDuplicatable) {
     LLVM_DEBUG(dbgs() << "  Not unrolling loop which contains non-duplicatable"
                       << " instructions.\n");
     return LoopUnrollResult::Unmodified;
   }
 
   // When optimizing for size, use LoopSize as threshold, to (fully) unroll
   // loops, if it does not increase code size.
   if (OptForSize)
-    UP.Threshold = std::max(UP.Threshold, LoopSize);
+    UP.Threshold = std::max(UP.Threshold, LoopSize + 1);
 
   if (NumInlineCandidates != 0) {
     LLVM_DEBUG(dbgs() << "  Not unrolling loop with inlinable calls.\n");
     return LoopUnrollResult::Unmodified;
   }
 
   // Find trip count and trip multiple if count is not available
   unsigned TripCount = 0;

llvm/test/Transforms/LoopUnroll/optsize-loop-size.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -loop-unroll -S < %s | FileCheck %s
+
+define i32 @test(i32 %a, i32 %b, i32 %c) optsize {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[REF_TMP:%.*]] = alloca [3 x i32], align 4
+; CHECK-NEXT:    [[TMP0:%.*]] = bitcast [3 x i32]* [[REF_TMP]] to i8*
+; CHECK-NEXT:    [[ARRAYINIT_BEGIN:%.*]] = getelementptr inbounds [3 x i32], [3 x i32]* [[REF_TMP]], i64 0, i64 0
+; CHECK-NEXT:    store i32 [[A:%.*]], i32* [[ARRAYINIT_BEGIN]], align 4
+; CHECK-NEXT:    [[ARRAYINIT_ELEMENT:%.*]] = getelementptr inbounds [3 x i32], [3 x i32]* [[REF_TMP]], i64 0, i64 1
+; CHECK-NEXT:    store i32 [[B:%.*]], i32* [[ARRAYINIT_ELEMENT]], align 4
+; CHECK-NEXT:    [[ARRAYINIT_ELEMENT1:%.*]] = getelementptr inbounds [3 x i32], [3 x i32]* [[REF_TMP]], i64 0, i64 2
+; CHECK-NEXT:    store i32 [[C:%.*]], i32* [[ARRAYINIT_ELEMENT1]], align 4
+; CHECK-NEXT:    [[ADD_PTR_I_I:%.*]] = getelementptr inbounds [3 x i32], [3 x i32]* [[REF_TMP]], i64 0, i64 3
+; CHECK-NEXT:    [[CMP_I_I_I3:%.*]] = icmp slt i32 [[A]], [[B]]
+; CHECK-NEXT:    [[SPEC_SELECT_I_I4:%.*]] = select i1 [[CMP_I_I_I3]], i32* [[ARRAYINIT_ELEMENT]], i32* [[ARRAYINIT_BEGIN]]
+; CHECK-NEXT:    [[INCDEC_PTR_I_I5:%.*]] = getelementptr inbounds [3 x i32], [3 x i32]* [[REF_TMP]], i64 0, i64 2
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[DOTPRE:%.*]] = load i32, i32* [[SPEC_SELECT_I_I4]], align 4
+; CHECK-NEXT:    [[DOTPRE2:%.*]] = load i32, i32* [[INCDEC_PTR_I_I5]], align 4
+; CHECK-NEXT:    [[CMP_I_I_I:%.*]] = icmp slt i32 [[DOTPRE]], [[DOTPRE2]]
+; CHECK-NEXT:    [[SPEC_SELECT_I_I:%.*]] = select i1 [[CMP_I_I_I]], i32* [[INCDEC_PTR_I_I5]], i32* [[SPEC_SELECT_I_I4]]
+; CHECK-NEXT:    [[INCDEC_PTR_I_I:%.*]] = getelementptr inbounds i32, i32* [[INCDEC_PTR_I_I5]], i64 1
+; CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* [[SPEC_SELECT_I_I]], align 4
+; CHECK-NEXT:    ret i32 [[TMP1]]
+;
+entry:
+  %ref.tmp = alloca [3 x i32], align 4
+  %0 = bitcast [3 x i32]* %ref.tmp to i8*
+  %arrayinit.begin = getelementptr inbounds [3 x i32], [3 x i32]* %ref.tmp, i64 0, i64 0
+  store i32 %a, i32* %arrayinit.begin, align 4
+  %arrayinit.element = getelementptr inbounds [3 x i32], [3 x i32]* %ref.tmp, i64 0, i64 1
+  store i32 %b, i32* %arrayinit.element, align 4
+  %arrayinit.element1 = getelementptr inbounds [3 x i32], [3 x i32]* %ref.tmp, i64 0, i64 2
+  store i32 %c, i32* %arrayinit.element1, align 4
+  %add.ptr.i.i = getelementptr inbounds [3 x i32], [3 x i32]* %ref.tmp, i64 0, i64 3
+  %cmp.i.i.i3 = icmp slt i32 %a, %b
+  %spec.select.i.i4 = select i1 %cmp.i.i.i3, i32* %arrayinit.element, i32* %arrayinit.begin
+  %incdec.ptr.i.i5 = getelementptr inbounds [3 x i32], [3 x i32]* %ref.tmp, i64 0, i64 2
+  br label %loop
+
+loop:          ; preds = %entry, %loop
+  %incdec.ptr.i.i7 = phi i32* [ %incdec.ptr.i.i5, %entry ], [ %incdec.ptr.i.i, %loop ]
+  %spec.select.i.i6 = phi i32* [ %spec.select.i.i4, %entry ], [ %spec.select.i.i, %loop ]
+  %.pre = load i32, i32* %spec.select.i.i6, align 4
+  %.pre2 = load i32, i32* %incdec.ptr.i.i7, align 4
+  %cmp.i.i.i = icmp slt i32 %.pre, %.pre2
+  %spec.select.i.i = select i1 %cmp.i.i.i, i32* %incdec.ptr.i.i7, i32* %spec.select.i.i6
+  %incdec.ptr.i.i = getelementptr inbounds i32, i32* %incdec.ptr.i.i7, i64 1
+  %cmp1.i.i = icmp eq i32* %incdec.ptr.i.i, %add.ptr.i.i
+  br i1 %cmp1.i.i, label %exit, label %loop
+
+exit:           ; preds = %loop
+  %1 = load i32, i32* %spec.select.i.i, align 4
+  ret i32 %1
+}