-- To avoid forgetting about this I post my bug here --
GVN, or probably memdep analysis, seem to think intrinsics are somewhat special
and consequently assume they cannot "effectively clobber" a memory location. It
might also be globals-mod-ref or something else that provides AA information.
The following example (Compiler Explorer) shows how an unknown function
prevents an invalid load hoist while an unknown intrinsic does not. Again, the preload
is not sound in a multi-thread environment.
opt -aa-pipeline=basic-aa,globals-aa -passes="require<globals-aa>,function(gvn<pre;load-pre;split-backedge-load-pre;memdep>)"
@G= internaladdrspace(3) globali32undef
declarevoid@llvm.sync()
definei32@sync_gvn_bug(i1%cond) nounwind{
entry:
br i1%cond, label%t, label%f
t:
store i3242, i32addrspace(3)* @G
br label%m
f:
br label%m
m:
call void@llvm.sync()
%r= load i32, i32addrspace(3)* @G
call void@llvm.sync()
ret i32%r
}
declarevoid@sync()
definei32@sync_gvn_bug_ok(i1%cond) nounwind{
entry:
br i1%cond, label%t, label%f
t:
store i3242, i32addrspace(3)* @G
br label%m
f:
br label%m
m:
call void@sync()
%r= load i32, i32addrspace(3)* @G
call void@sync()
ret i32%r
}
Help with fixing this is much appreciated!
~ Johannes