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[PM/LoopUnswitch] Detect irreducible control flow within loops and skip unswitching non-trivial edges.

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Summary:
This fixes the bug pointed out in review with non-trivial unswitching.

This also provides a basis that should make it pretty easy to finish
fleshing out a routine to scan an entire function body for irreducible
control flow, but this patch remains minimal for disabling loop
unswitch.

Reviewers: sanjoy, fedor.sergeev

Subscribers: mcrosier, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D45754

llvm-svn: 330357
This commit is contained in:
Chandler Carruth 2018-04-19 18:44:25 +00:00
parent 52289d96c4
commit a806fbf4b8
2 changed files with 43 additions and 0 deletions
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13
lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
View File

@ -17,9 +17,11 @@
#include "llvm/ADT/Statistic.h" #include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h" #include "llvm/ADT/Twine.h"
#include "llvm/Analysis/AssumptionCache.h" #include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/CodeMetrics.h" #include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/LoopAnalysisManager.h" #include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopInfo.h" #include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/LoopPass.h" #include "llvm/Analysis/LoopPass.h"
#include "llvm/IR/BasicBlock.h" #include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h" #include "llvm/IR/Constant.h"
@ -1938,6 +1940,17 @@ unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI, AssumptionCache &AC,
if (UnswitchCandidates.empty()) if (UnswitchCandidates.empty())
return Changed; return Changed;
// Check if there are irreducible CFG cycles in this loop. If so, we cannot
// easily unswitch non-trivial edges out of the loop. Doing so might turn the
// irreducible control flow into reducible control flow and introduce new
// loops "out of thin air". If we ever discover important use cases for doing
// this, we can add support to loop unswitch, but it is a lot of complexity
// for what seems little or no real world benifit.
LoopBlocksRPO RPOT(&L);
RPOT.perform(&LI);
if (containsIrreducibleCFG<const BasicBlock *>(RPOT, LI))
return Changed;
DEBUG(dbgs() << "Considering " << UnswitchCandidates.size() DEBUG(dbgs() << "Considering " << UnswitchCandidates.size()
<< " non-trivial loop invariant conditions for unswitching.\n"); << " non-trivial loop invariant conditions for unswitching.\n");

30
test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll
View File

@ -2350,3 +2350,33 @@ loop_exit:
; CHECK-NEXT: %[[LCSSA:.*]] = phi i32 [ %[[V]], %loop_begin ] ; CHECK-NEXT: %[[LCSSA:.*]] = phi i32 [ %[[V]], %loop_begin ]
; CHECK-NEXT: ret i32 %[[LCSSA]] ; CHECK-NEXT: ret i32 %[[LCSSA]]
} }
; Negative test: we do not switch when the loop contains unstructured control
; flows as it would significantly complicate the process as novel loops might
; be formed, etc.
define void @test_no_unswitch_unstructured_cfg(i1* %ptr, i1 %cond) {
; CHECK-LABEL: @test_no_unswitch_unstructured_cfg(
entry:
br label %loop_begin
loop_begin:
br i1 %cond, label %loop_left, label %loop_right
loop_left:
%v1 = load i1, i1* %ptr
br i1 %v1, label %loop_right, label %loop_merge
loop_right:
%v2 = load i1, i1* %ptr
br i1 %v2, label %loop_left, label %loop_merge
loop_merge:
%v3 = load i1, i1* %ptr
br i1 %v3, label %loop_latch, label %loop_exit
loop_latch:
br label %loop_begin
loop_exit:
ret void
}