[Bug rtl-optimization/40838] gcc shouldn't assume that the stack is aligned

[hjl dot tools at gmail dot com]

------- Comment #51 from hjl dot tools at gmail dot com  2009-09-26 16:15 -------
(In reply to comment #50)
> 
> I am saying that the patch could be included in 4.4 as a quick fix and that 4.5
> needs stack alignment redesign. You can't redesign it by incrementally testing
> against a set of examples. You (or someone else) need to sit down, draft the
> rules for propagation of preferred and enforced alignment across types down to
> the stack frame and code it. If you understand what you're doing, you can
> create test examples on your own. If you don't understand, then don't hack it
> at all.
>

The stack realignment works correctly as designed. The problems come from

1. Gcc generates code for 16byte-aligned incoming stack.
2. Gcc generates 16byte-aligned outgoing stack.
3. SSE variables segfault if they aren't 16byte aligned.
4. Some incoming stacks aren't aligned at 16byte, which violates #1 and
may cause #3.

We have to do #2 since many existing binaries need 16byte-aligned
incoming stack. If we assume incoming stack is 4byte aligned, we
have to realign stack for every function due to #2, which isn't
acceptable. To accommodate #4, which isn't strictly required, and
avoid #3 with minimum stack realignment, we need to list all #4
cases which lead to #3. Then we can investigate how to address them.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838