https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345
Bug ID: 88345
Summary: -Os overrides -finline-functions=N on the command line
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
The -Os effect on -falign-functions is documented in a confusing or even
contradictory way:
Optimize for size. -Os enables all -O2 optimizations except those that
often increase code size:
-falign-functions -falign-jumps
-falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stc
It also enables -finline-functions, causes the compiler to tune for code
size rather than execution speed, and performs further optimizations designed
to reduce code size.
The first sentence implies -Os is doesn't enable -finline-functions, but the
second sentence contradicts it.
The documentation for -finline-functions=n doesn't help. It just says:
If n is not specified or is zero, use a machine-dependent default.
I haven't been able to find a way to determine the machine-dependent default
except by testing.
An experiment with an x86_64 compiler shows that -Os overrides whatever
-falign-functions option is specified on the command line. Curiously, though,
with -O2 GCC does honor the -falign-functions=n setting.
Clang, on the other hand, does the opposite: it honors the -falign-functions=n
setting with -Os and ignores it with -O2.
$ cat u.c && gcc -Os -c -Wall -falign-functions=2 u.c && objdump -d u.o
void f (void) { }
void g (void) { }
u.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <f>:
0: c3 retq
0000000000000001 <g>:
1: c3 retq
