[Bug target/39942] Nonoptimal code - leaveq; xchg %ax,%ax; retq

[hjl dot tools at gmail dot com]

------- Comment #40 from hjl dot tools at gmail dot com  2009-05-15 14:35 -------
(In reply to comment #37)
> This patch looks very wrong.  It assumes that min_insn_size gives exact insn
> sizes (current min_insn_size is very far from that, but even get_attr_length
> isn't exact), doesn't take into account label alignments nor branch shortening
> which can change the insn sizes afterwards and assumes that a p2align always
> aligns to 16 bytes (it does not).
> While the previous algorithm works with estimated 16 consecutive bytes rather
> than 16 byte pages 0xXXXX0 ... 0xXXXF, that's because during machine reorg
> you simply can't know in most cases where exactly the 16 byte page will start,
> so you assume it can start (almost) anywhere (and use .p2align behavior to
> align when needed).
> 

There is no perfect solution here. Let's list pros/cons:

The current algorithm:

pros:

1. Very conservative. Catch most of 4 branches in 16byte windows.

Cons:

1. It works on 16byte window, not 16byte page.
2. When it gets wrong, it increases code sizes by adding unnecessary
nops.

My proposal:

Pros:

1. Work on 16byte page.
2. Even if it gets wrong, it doesn't increase code size.

Cons:

1. Rely on inaccurate instruction length data.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39942