This issue is well-known in research/scientific software. The problem of
compiler hang or RAM overconsumption is actually not about the routine
size, but about too complicated control flow. When optimizing, the compiler
traverses the control flow graph, which may have the misfortune to explode
in terms of complexity. So you may want to check whether your routine
heavily deploys nested cascades of "if ... else" or goto-s. That is, the
routine size is not a good metric to catch this behavior. GCC may rather
attempt "reversible" strategy of optimizations to stop and undo those that
get beyond a certain threshold.
Kind regards,
- Dmitry.
чт, 19 дек. 2019 г. в 17:38, Qing Zhao <qing.z...@oracle.com>:
> Hi,
>
> When using GCC to compile a very large routine with -O2, it failed with
> out of memory during run time. (O1 is Okay)
>
> As I checked within gdb, when “cc1” was consuming around 95% of the
> memory, it’s at :
>
> (gdb) where
> #0 0x0000000000ddbcb3 in df_chain_create (src=0x631006480f08,
> dst=0x63100f306288) at ../../gcc-8.2.1-20180905/gcc/df-problems.c:2267
> #1 0x0000000000dddd1a in df_chain_create_bb_process_use (
> local_rd=0x7ffc109bfaf0, use=0x63100f306288, top_flag=0)
> at ../../gcc-8.2.1-20180905/gcc/df-problems.c:2441
> #2 0x0000000000dde5a7 in df_chain_create_bb (bb_index=16413)
> at ../../gcc-8.2.1-20180905/gcc/df-problems.c:2490
> #3 0x0000000000ddeaa9 in df_chain_finalize (all_blocks=0x63100097ac28)
> at ../../gcc-8.2.1-20180905/gcc/df-problems.c:2519
> #4 0x0000000000dbe95e in df_analyze_problem (dflow=0x60600027f740,
> blocks_to_consider=0x63100097ac28, postorder=0x7f23761f1800,
> n_blocks=40768) at ../../gcc-8.2.1-20180905/gcc/df-core.c:1179
> #5 0x0000000000dbedac in df_analyze_1 ()
> ….
>
> The routine that was compiled is very big, has about 119258 lines of code.
> I suspected that GCC’s data flow analysis might not handle very large
> routine very well, consume too much memory, therefore out of memory for
> very big routines.
>
> Currently, I found one GCC’s source level pragma,
>
> #pragma GCC optimize ("O1”)
>
> And added it before the large routine (also added another one #pragma GCC
> reset_options after the routine), this workaround the out of memory issue
> for now.
>
> However, manually locating large routines is time consuming, I am
> wondering whether GCC can automatically detect large routines and lower the
> optimization for those
> Routines automatically? Or is there any internal parameters inside GCC’s
> data flow analysis that compute the complexity of the routine, if it’s very
> big, then will turn off
> The aggressive analysis automatically? Or any option provided to end user
> to control the aggressive data flow manually ?
>
>
> Thanks a lot for any help.
>
> Qing
