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Does gcc automatically lower optimization level for very large routines?
- From: Qing Zhao <QING dot ZHAO at ORACLE dot COM>
- To: gcc at gcc dot gnu dot org
- Date: Thu, 19 Dec 2019 10:37:52 -0600
- Subject: Does gcc automatically lower optimization level for very large routines?
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 :
#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)
#2 0x0000000000dde5a7 in df_chain_create_bb (bb_index=16413)
#3 0x0000000000ddeaa9 in df_chain_finalize (all_blocks=0x63100097ac28)
#4 0x0000000000dbe95e in df_analyze_problem (dflow=0x60600027f740,
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.