Add llvm.sideeffect to potential infinite loops and recursions

LLVM assumes that a thread will eventually cause side effect. This is
not true in Rust if a loop or recursion does nothing in its body,
causing undefined behavior even in common cases like `loop {}`.
Inserting llvm.sideeffect fixes the undefined behavior.

As a micro-optimization, only insert llvm.sideeffect when jumping back
in blocks or calling a function.

A patch for LLVM is expected to allow empty non-terminate code by
default and fix this issue from LLVM side.

https://github.com/rust-lang/rust/issues/28728

1 files changed, 5 insertions, 1 deletions

diff --git a/src/test/codegen/repeat-trusted-len.rs b/src/test/codegen/repeat-trusted-len.rs
index 87f29f6047c..40399e8f76f 100644
--- a/src/test/codegen/repeat-trusted-len.rs
+++ b/src/test/codegen/repeat-trusted-len.rs
@@ -14,6 +14,10 @@ pub fn helper(_: usize) {
 // CHECK-LABEL: @repeat_take_collect
 #[no_mangle]
 pub fn repeat_take_collect() -> Vec<u8> {
-// CHECK: call void @llvm.memset.p0i8.[[USIZE]](i8* {{(nonnull )?}}align 1 %{{[0-9]+}}, i8 42, [[USIZE]] 100000, i1 false)
+// FIXME: At the time of writing LLVM transforms this loop into a single
+// `store` and then a `memset` with size = 99999. The correct check should be:
+//        call void @llvm.memset.p0i8.[[USIZE]](i8* {{(nonnull )?}}align 1 %{{[a-z0-9.]+}}, i8 42, [[USIZE]] 100000, i1 false)
+
+// CHECK: call void @llvm.memset.p0i8.[[USIZE]](i8* {{(nonnull )?}}align 1 %{{[a-z0-9.]+}}, i8 42, [[USIZE]] 99999, i1 false)
     iter::repeat(42).take(100000).collect()
 }