FRE may run out of memory

[dxq]

Richard Biener-2 wrote
> On Sat, Feb 8, 2014 at 8:29 AM, dxq <

> ziyan01@

> > wrote:
>> hi all,
>>
>> We found that gcc would run out of memory on Windows when compiling a
>> *big*
>> function (100000 lines).
>>
>> More investigation shows that gcc crashes at the function
>> *compute_avail*,
>> in tree-fre pass.  *compute_avail* collects information from basic
>> blocks,
>> so memory is allocated to record informantion.
>> However, if there are huge number of basic blocks,  the memory would be
>> exhausted and gcc would crash down, especially for Windows PC, only 2G or
>> 4G
>> memory generally. It's ok On linux, and *compute_avail* allocates *2.4G*
>> memory. I guess some optimization passes in gcc like FRE didn't consider
>> the
>> extreme
>> case.
> 
> This was fixed for GCC 4.8, FRE no longer uses compute_avail (but PRE
> still does).
> Basically GCC 4.8 should (at -O1) compile most extreme cases just fine.
> 
> Richard.

hi, Richard,

More  investigation shows that 
1, loop related passes take more compiling time and memory, especially
pass_rtl_move_loop_invariants, lim, 
  and at least lim on tree will impact a lot to the following passes. 
2, ira will take more than 20g memory in function *create_loop_tree_nodes*,
because ira chooses 'mixed' 
  or 'all' region when optimize level. 
3, sms pass always creats ddgs for all loops in compiled function, then does
sms optimization for all loops, 
  and finally frees ddgs. If there are huge number of loops, sms may crash
when creating ddgs because of 
  running out of memory.

The passes above , should someone confirm about memory pressure problem?

Thanks for your reply!

danxiaoqiang



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