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

1 files changed, 9 insertions, 0 deletions

diff --git a/test/CodeGen/Generic/intrinsics.ll b/test/CodeGen/Generic/intrinsics.ll
index 4d04786a58d..6a51d2d371b 100644
--- a/test/CodeGen/Generic/intrinsics.ll
+++ b/test/CodeGen/Generic/intrinsics.ll
@@ -45,3 +45,12 @@ define i8* @barrier(i8* %p) {
         %q = call i8* @llvm.invariant.group.barrier(i8* %p)
         ret i8* %q
 }
+
+; sideeffect
+
+declare void @llvm.sideeffect()
+
+define void @test_sideeffect() {
+    call void @llvm.sideeffect()
+    ret void
+}