https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64827
Bug ID: 64827
Summary: LTO doesn't propogate/optimise initialised global
variable value passed by reference
Product: gcc
Version: 4.9.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: lto
Assignee: unassigned at gcc dot gnu.org
Reporter: justynbutlerspam at gmail dot com
Sorry if this is already well-known or I'm misinformed, but I couldn't find any
reference to it.
Consider the following code:
#include <stdio.h>
int doublefunc (int *input) {
return *input *2;
}
int foo = 3;
int main(void) {
if ( doublefunc(&foo) == 2 )
printf( "operation result was 2" );
}
Compiled with:
gcc -ffunction-sections -fdata-sections -O3 -flto -Xlinker --gc-sections -O3
-flto -o main main.c
With the doublefunc defined in the same translation unit as above, the compiler
correctly removes the printf statement as dead code (unless foo is initialised
as 1).
But if the function is put into a separate file and compiled with the same
options (including -flto), the optimisation doesn't happen. This seems to me
like an LTO failure.
I've found that in several situations the optimisation will successfully occur
even when the function is in a separate unit:
* if the global variable is initialised with const
* if the value of the variable is set inside main
* if a second (global or otherwise) variable is assigned the value of the first
variable inside main and that is passed to doublefunc
The last one makes it appear particularly strange, since using the following
(with doublefunc in a separate file) will lead the compiler to optimise
correctly:
int foo = 3;
int bar;
int main(void) {
bar=foo;
if ( doublefunc(&bar) == 2 )
printf( "operation result was 2" );
}
The problem only seems to affect variables passed by reference, the
optimisation works if the variable is passed by value instead.
I've tested this with gcc 4.8.2 and also arm-none-eabi-gcc 4.9.3 on Ubuntu
14.04 64-bit.
Am I missing something fundamental or is it a bug?
