https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104468
Bug ID: 104468
Summary: with -O -g, quadratic compile time of function with
__attribute__(("00")) that passes large structs by
value
Product: gcc
Version: 11.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: erik.carstensen at intel dot com
Target Milestone: ---
Created attachment 52392
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52392&action=edit
Reproducer
If a function passes many large structs by value to another function, then you
get quadratic compile performance (O(n^2)) if the file is compiled with -O -g,
but the function is annotated with __attribute__((optimize("O0"))).
Compile time seems (approximately) quadratic independently in the number of
calls, in the number of struct function arguments, and in the size of the
struct. In other words, quadratic in the total size of passed values.
It compiles instantaneously (30s -> 0.1s) if I remove the __attribute__, or -g,
or -O, or if the struct size is changed to <=16 bytes or >=81 bytes.
It's still slow if I pass
-O -fno-auto-inc-dec -fno-branch-count-reg -fno-combine-stack-adjustments
-fno-compare-elim -fno-cprop-registers -fno-dce -fno-defer-pop -fno-dse
-fno-forward-propagate -fno-guess-branch-probability -fno-if-conversion
-fno-if-conversion2 -fno-inline-functions-called-once -fno-ipa-modref
-fno-ipa-profile -fno-ipa-pure-const -fno-ipa-reference
-fno-ipa-reference-addressable -fno-merge-constants -fno-move-loop-invariants
-fno-omit-frame-pointer -fno-reorder-blocks -fno-shrink-wrap
-fno-shrink-wrap-separate -fno-split-wide-types -fno-ssa-backprop
-fno-ssa-phiopt -fno-tree-bit-ccp -fno-tree-ccp -fno-tree-ch
-fno-tree-coalesce-vars -fno-tree-copy-prop -fno-tree-dce
-fno-tree-dominator-opts -fno-tree-dse -fno-tree-forwprop -fno-tree-fre
-fno-tree-phiprop -fno-tree-pta -fno-tree-scev-cprop -fno-tree-sink
-fno-tree-slsr -fno-tree-sra -fno-tree-ter -fno-unit-at-a-time
... which is documented to be the same as -O0.
This happens with native gcc from Fedora 34:
$ gcc --version
gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1)
$ uname -a
Linux ecarsten-mobl1.ger.corp.intel.com 5.15.12-100.fc34.x86_64 #1 SMP Wed Dec
29 15:21:44 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
Also reproduced with gcc 6.4.
Command line:
$ gcc -g -O1 -c foo.c
or alternatively (to bypass ccache on my system):
$ /usr/libexec/gcc/x86_64-redhat-linux/11/cc1 -quiet foo.c -quiet -dumpbase
foo.c -dumpbase-ext .c -mtune=generic -march=x86-64 -g -O0 -o /tmp/ccFglVbD.s
This causes performance issues in C code generated by the DML compiler
(https://github.com/intel/device-modeling-language)
