[llvm-dev] Recent -Os code size regressions

[Steve King via llvm-dev]

On Fri, Nov 20, 2015 at 5:06 PM, James Molloy <james at jamesmolloy.co.uk>
wrote:
>
> Hi,
>
> We'd need to look precisely at what's causing the code size bloat. The
midend commit pointed out by Steve shouldn't cause bloat in and of itself -
it should reduce code size. It removes a load of stores and branches.
>
> I know a backend change I made to ARM isn't behaving as well as it could,
and I have patches to fix that. Speculatively reverting midend patches
isn't the best way to approach this, in my opinion! :)
>

​​
For i586, the effect of r252152 seems to cause cmoves instead of branches.
​  Code size increase is +35% for i586.
Unfortunately the object files are wildly different in a way that does not
seem to occur in other workloads.  I tried to clip a concise before and
after case.

Before
​:
As a reference point, I found OR $0x408 and OR​ $0x810 in close proximity.


 278: 81 ca 10 08 00 00     or     $0x810,%edx
 27e: 89 10                 mov    %edx,(%eax)
 280: f6 c1 40
​          ​
    test   $0x40,%cl
 283: 74 08                 je     28d <t_run_test+0x28d>
 285: 81 ca 08 04 00 00     or     $0x408,%edx
 28b: 89 10                 mov    %edx,(%eax)
 28d: 84 c9                 test   %cl,%cl
 28f: 0f 89 34 01 00 00     jns    3c9 <t_run_test+0x3c9>


After
​ r252152:​

Note that the OR $0x408 and OR $0x810 come
​now ​
in reverse order.


35d: 81 c9 08 04 00 00     or     $0x408,%ecx
363: 89 4c 24 28           mov    %ecx,0x28(%esp)
367: 89 df                 mov    %ebx,%edi
369: 83 e7 10
​        ​
      and    $0x10,%edi
36c: 89 7c 24 20           mov    %edi,0x20(%esp)
370: 0f 45 d1
​           ​
   cmovne %ecx,%edx
373: 89 d7                 mov    %edx,%edi
375: 81 cf 10 08 00 00     or     $0x810,%edi
37b: 89 7c 24 14           mov    %edi,0x14(%esp)
37f: 89 d9                 mov    %ebx,%ecx
381: 83 e1 20
​       ​
       and    $0x20,%ecx
384: 89 4c 24 1c           mov    %ecx,0x1c(%esp)
388: 0f 45 d7
​        ​
      cmovne %edi,%edx
38b: 89 d7                 mov    %edx,%edi


HTH,
-steve​
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