http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58920
Bug ID: 58920
Summary: Overeager optimization based on TREE_THIS_NOTRAP
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jason at gcc dot gnu.org
Created attachment 31108
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31108&action=edit
Patch to set TREE_THIS_NOTRAP for C++ references
The TREE_THIS_NOTRAP macro came up in email the other day, and it seemed to me
that it would be useful to set on C++ references, since they are required to
refer to objects; trying to read from a null reference gives undefined
behavior. So I tried the attached patch. But it breaks all the ext_pointer
tests in libstdc++.
Basically, what's happening is that there is a code path which is never taken
which leads to an explicit null dereference. The optimizers see this happening
within a loop and decide to hoist it out of the loop. So now it is executed
before the loop starts, and causes a SEGV.
Is this an appropriate optimization?
