>Number:         10076
>Category:       optimization
>Synopsis:       Use of function call return value
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Fri Mar 14 08:16:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Gábor Lóki
>Release:        GCC: (GNU) 3.3 20030303 (prerelease)
>Organization:
>Environment:
BUILD & HOST: Linux 2.4.20 i686 unknown
TARGET: arm-unknown-elf
>Description:
After a function call where the return value is stored in a hard register, GCC moves this value into a soft register. After that if an instruction uses this value, the soft register will be used.
In some cases the optimization algorithms aren't able to recognize that the soft and hard registers which hold the return value are the same. This results in an unnecessary move instruction for each arithmetic instruction that uses the return value. (In some other cases the unnecessary move is optimized out by some of the algorithms.)
In the expamle attached, in function foo1 there is an unnecessary move, while in the case of function foo2 there isn't such.
>How-To-Repeat:
arm-elf-gcc -S -g0 -Os function-call-return-value.i

int bongo(int);
int j;
int foo1()
{
  j=bongo(44);
}

void foo2()
{
  j=bongo(44);
} 
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
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