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LoopRotate: don't rotate loops where the header block branches to a dead-end block. #78781

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Merged
merged 1 commit into from
Jan 22, 2025
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LoopRotate: don't rotate loops where the header block branches to a dead-end block. #78781

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10 changes: 10 additions & 0 deletions lib/SILOptimizer/LoopTransforms/LoopRotate.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,7 @@
#include "swift/SIL/SILBuilder.h"
#include "swift/SIL/SILInstruction.h"
#include "swift/SILOptimizer/Analysis/Analysis.h"
#include "swift/SILOptimizer/Analysis/DeadEndBlocksAnalysis.h"
#include "swift/SILOptimizer/Analysis/DominanceAnalysis.h"
#include "swift/SILOptimizer/Analysis/LoopAnalysis.h"
#include "swift/SILOptimizer/PassManager/Passes.h"
Expand Down Expand Up @@ -374,6 +375,15 @@ static bool rotateLoop(SILLoop *loop, DominanceInfo *domInfo,
return false;
}

// Incomplete liveranges in the dead-end exit block can cause a missing adjacent
// phi-argument for a re-borrow if there is a borrow-scope is in the loop.
// But even when we have complete lifetimes, it's probably not worth rotating
// a loop where the header block branches to a dead-end block.
auto *deBlocks = pm->getAnalysis<DeadEndBlocksAnalysis>()->get(exit->getParent());
if (deBlocks->isDeadEnd(exit)) {
return false;
}

// We don't want to rotate such that we merge two headers of separate loops
// into one. This can be turned into an assert again once we have guaranteed
// preheader insertions.
Expand Down
37 changes: 37 additions & 0 deletions test/SILOptimizer/looprotate_ossa.sil
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Expand Up @@ -18,6 +18,7 @@ class Bar {
@objc func foo_objc()
}

sil [ossa] @getBar : $@convention(thin) () -> @owned Bar
sil [ossa] @_TFC4main3Bar3foofS0_FT_T_ : $@convention(method) (@guaranteed Bar) -> ()
sil [ossa] @_TFC4main3Bar3boofS0_FT_T_ : $@convention(method) (@guaranteed Bar) -> Int64
sil [ossa] @_TFC4main3Bar3foo_objcfS0_FT_T_ : $@convention(objc_method) (Bar) -> ()
Expand Down Expand Up @@ -713,3 +714,39 @@ bb14:
return %27
}

// Incomplete liveranges in dead-end exit blocks can cause a missing phi in the rotated loop.
// CHECK-LABEL: sil [ossa] @incomplete_liverange_in_dead_end_exit :
// CHECK: bb1:
// CHECK: apply
// CHECK: bb2:
// CHECK-LABEL: } // end sil function 'incomplete_liverange_in_dead_end_exit'
sil [ossa] @incomplete_liverange_in_dead_end_exit : $@convention(thin) () -> () {
bb0:
br bb1

bb1:
%1 = function_ref @getBar : $@convention(thin) () -> @owned Bar
%2 = apply %1() : $@convention(thin) () -> @owned Bar
%3 = begin_borrow %2
cond_br undef, bb3, bb2

bb2:
br bb4

bb3:
end_borrow %3
unreachable

bb4:
end_borrow %3
destroy_value %2
cond_br undef, bb6, bb5

bb5:
br bb1

bb6:
%12 = tuple ()
return %12
}