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| author | Dan Gohman <dan433584@gmail.com> | 2017-11-08 21:59:51 +0000 |
|---|---|---|
| committer | Dan Gohman <dan433584@gmail.com> | 2017-11-08 21:59:51 +0000 |
| commit | b5e0bec282c01ed9e075a9587e1c4274c3d939fe (patch) | |
| tree | ea1a52ffb4a64ddb467bd7304eb55ce642690c78 /docs | |
| parent | bfc1e953bcc9264c0873a0a8546d487b407e3c76 (diff) | |
Add an @llvm.sideeffect intrinsic
This patch implements Chandler's idea [0] for supporting languages that
require support for infinite loops with side effects, such as Rust, providing
part of a solution to bug 965 [1].
Specifically, it adds an `llvm.sideeffect()` intrinsic, which has no actual
effect, but which appears to optimization passes to have obscure side effects,
such that they don't optimize away loops containing it. It also teaches
several optimization passes to ignore this intrinsic, so that it doesn't
significantly impact optimization in most cases.
As discussed on llvm-dev [2], this patch is the first of two major parts.
The second part, to change LLVM's semantics to have defined behavior
on infinite loops by default, with a function attribute for opting into
potential-undefined-behavior, will be implemented and posted for review in
a separate patch.
[0] http://lists.llvm.org/pipermail/llvm-dev/2015-July/088103.html
[1] https://bugs.llvm.org/show_bug.cgi?id=965
[2] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118632.html
Differential Revision: https://reviews.llvm.org/D38336
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317729 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/LangRef.rst | 30 |
1 files changed, 30 insertions, 0 deletions
diff --git a/docs/LangRef.rst b/docs/LangRef.rst index 8cbed7d87d1..b097230591f 100644 --- a/docs/LangRef.rst +++ b/docs/LangRef.rst @@ -14267,6 +14267,36 @@ not overflow at link time under the medium code model if ``x`` is an a constant initializer folded into a function body. This intrinsic can be used to avoid the possibility of overflows when loading from such a constant. +'``llvm.sideeffect``' Intrinsic +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Syntax: +""""""" + +:: + + declare void @llvm.sideeffect() inaccessiblememonly nounwind + +Overview: +""""""""" + +The ``llvm.sideeffect`` intrinsic doesn't perform any operation. Optimizers +treat it as having side effects, so it can be inserted into a loop to +indicate that the loop shouldn't be assumed to terminate (which could +potentially lead to the loop being optimized away entirely), even if it's +an infinite loop with no other side effects. + +Arguments: +"""""""""" + +None. + +Semantics: +"""""""""" + +This intrinsic actually does nothing, but optimizers must assume that it +has externally observable side effects. + Stack Map Intrinsics -------------------- |