7b78d3920c
The description of `isGuaranteedToExecute` does not correspond to its implementation. According to description, it should return `true` if an instruction is executed under the assumption that its loop is *entered*. However there is a sophisticated alrogithm inside that tries to prove that the instruction is executed if the loop is *exited*, which is not the same thing for infinite loops. There is an attempt to protect from dealing with infinite loops by prohibiting loops without exit blocks, however an infinite loop can have exit blocks. As result of that, MustExecute can falsely consider some blocks that are never entered as mustexec, and LICM can hoist dangerous instructions out of them basing on this fact. This may introduce UB to programs which did not contain it initially. This patch removes the problematic algorithm and replaced it with a one which tries to prove what is required in description. Differential Revision: https://reviews.llvm.org/D50558 Reviewed By: reames llvm-svn: 339984
133 lines
4.6 KiB
LLVM
133 lines
4.6 KiB
LLVM
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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; RUN: opt -S -basicaa -licm < %s | FileCheck %s
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; RUN: opt -aa-pipeline=basic-aa -passes='require<opt-remark-emit>,loop(licm)' -S %s | FileCheck %s
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; Make sure we don't hoist the unsafe division to some executable block.
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define void @test_impossible_exit_in_untaken_block(i32 %a, i32 %b, i32* %p) {
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; CHECK-LABEL: @test_impossible_exit_in_untaken_block(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: br label [[LOOP:%.*]]
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; CHECK: loop:
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; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ]
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; CHECK-NEXT: br i1 false, label [[NEVER_TAKEN:%.*]], label [[BACKEDGE]]
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; CHECK: never_taken:
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; CHECK-NEXT: [[DIV:%.*]] = sdiv i32 [[A:%.*]], [[B:%.*]]
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; CHECK-NEXT: store i32 [[DIV]], i32* [[P:%.*]]
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; CHECK-NEXT: br i1 true, label [[BACKEDGE]], label [[EXIT:%.*]]
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; CHECK: backedge:
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; CHECK-NEXT: [[IV_NEXT]] = add i32 [[IV]], 1
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; CHECK-NEXT: br label [[LOOP]]
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; CHECK: exit:
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; CHECK-NEXT: ret void
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;
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entry:
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br label %loop
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loop:
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%iv = phi i32 [ 0, %entry ], [ %iv.next, %backedge ]
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br i1 false, label %never_taken, label %backedge
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never_taken:
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%div = sdiv i32 %a, %b
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store i32 %div, i32* %p
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br i1 true, label %backedge, label %exit
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backedge:
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%iv.next = add i32 %iv, 1
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br label %loop
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exit:
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ret void
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}
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; The test above is UB in C++, because there is a requirement that any
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; thead should eventually terminate, execute volatile access operation, call IO
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; or synchronize. In spite of that, the behavior in the test above *might* be
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; correct. This one is equivalent to the test above, but it has a volatile
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; memory access in the loop's mustexec block, so the compiler no longer has a
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; right to assume that it must terminate. Show that the same problem persists,
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; and that it was a bug and not a cool optimization based on loop infinity.
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; By the moment when this test was added, it was accidentally correct due to
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; reasons not directly related to this piece of logic. Make sure that it keeps
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; correct in the future.
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define void @test_impossible_exit_in_untaken_block_no_ub(i32 %a, i32 %b, i32* noalias %p, i32* noalias %vp) {
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; CHECK-LABEL: @test_impossible_exit_in_untaken_block_no_ub(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: br label [[LOOP:%.*]]
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; CHECK: loop:
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; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ]
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; CHECK-NEXT: [[TMP0:%.*]] = load volatile i32, i32* [[VP:%.*]]
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; CHECK-NEXT: br i1 false, label [[NEVER_TAKEN:%.*]], label [[BACKEDGE]]
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; CHECK: never_taken:
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; CHECK-NEXT: [[DIV:%.*]] = sdiv i32 [[A:%.*]], [[B:%.*]]
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; CHECK-NEXT: store i32 [[DIV]], i32* [[P:%.*]]
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; CHECK-NEXT: br i1 true, label [[BACKEDGE]], label [[EXIT:%.*]]
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; CHECK: backedge:
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; CHECK-NEXT: [[IV_NEXT]] = add i32 [[IV]], 1
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; CHECK-NEXT: br label [[LOOP]]
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; CHECK: exit:
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; CHECK-NEXT: ret void
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;
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entry:
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br label %loop
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loop:
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%iv = phi i32 [ 0, %entry ], [ %iv.next, %backedge ]
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load volatile i32, i32* %vp
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br i1 false, label %never_taken, label %backedge
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never_taken:
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%div = sdiv i32 %a, %b
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store i32 %div, i32* %p
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br i1 true, label %backedge, label %exit
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backedge:
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%iv.next = add i32 %iv, 1
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br label %loop
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exit:
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ret void
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}
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; Same as above, but the volatile access is in mustexecute backedge block. The
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; loop is no longer "finite by specification", make sure we don't hoist sdiv
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; from it no matter how general the MustThrow analysis is.
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define void @test_impossible_exit_in_untaken_block_no_ub_2(i32 %a, i32 %b, i32* noalias %p, i32* noalias %vp) {
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; CHECK-LABEL: @test_impossible_exit_in_untaken_block_no_ub_2(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: br label [[LOOP:%.*]]
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; CHECK: loop:
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; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ]
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; CHECK-NEXT: br i1 false, label [[NEVER_TAKEN:%.*]], label [[BACKEDGE]]
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; CHECK: never_taken:
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; CHECK-NEXT: [[DIV:%.*]] = sdiv i32 [[A:%.*]], [[B:%.*]]
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; CHECK-NEXT: store i32 [[DIV]], i32* [[P:%.*]]
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; CHECK-NEXT: br i1 true, label [[BACKEDGE]], label [[EXIT:%.*]]
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; CHECK: backedge:
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; CHECK-NEXT: [[IV_NEXT]] = add i32 [[IV]], 1
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; CHECK-NEXT: [[TMP0:%.*]] = load volatile i32, i32* [[VP:%.*]]
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; CHECK-NEXT: br label [[LOOP]]
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; CHECK: exit:
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; CHECK-NEXT: ret void
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;
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entry:
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br label %loop
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loop:
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%iv = phi i32 [ 0, %entry ], [ %iv.next, %backedge ]
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br i1 false, label %never_taken, label %backedge
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never_taken:
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%div = sdiv i32 %a, %b
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store i32 %div, i32* %p
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br i1 true, label %backedge, label %exit
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backedge:
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%iv.next = add i32 %iv, 1
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load volatile i32, i32* %vp
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br label %loop
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exit:
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ret void
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}
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