Re: constant folding with constant function pointers in GCC 4.1.x

[Brian Dessent <brian at dessent dot net>]

"Frankel, Bob" wrote:

> volatile int OUT;
> 
> void foo()
> {
>     OUT = 100;
> }
> 
> void (*const FOOPTR)() = foo;
> 
> int main()
> {
>     FOOPTR();   /* remains an indirect call to foo */
>     return 0;
> }
> 
> i would have expected the indirect call to fold away into an assignment
> of 100 to OUT, as in the other examples.  i have seen this optimization
> occur on other compilers supporting whole-program optimization, and was
> wondering if there is something else i need to add to either my source
> file and/or my makefile (where i am already passing -O3 and
> -fwhole-program)

With 4.3 (not sure it matters) and a little modification to get rid of
volatile I was able to get it to optimize away the call through the
function pointer, resulting in just a regular function call, which I
would have thought would have been inlined under other circumstances,
but no matter what I tried it wouldn't inline it:

int foo(int i)
{
    return i + 42;
}

int (*FOOPTR)(int) = foo;

int main(int argc, char **argv)
{
    return FOOPTR(argc);
}

With -O3 -fwhole-program -fomit-frame-pointer, I get:

        .text
        .p2align 4,,15
        .def    _foo;   .scl    3;      .type   32;     .endef
_foo:
        movl    4(%esp), %eax    # i, tmp60
        addl    $42, %eax        #, tmp60
        ret
        .def    ___main;        .scl    2;      .type   32;     .endef
        .p2align 4,,15
.globl _main
        .def    _main;  .scl    2;      .type   32;     .endef
_main:
        leal    4(%esp), %ecx    #,
        andl    $-16, %esp       #,
        pushl   -4(%ecx)         #
        subl    $12, %esp        #,
        movl    %ecx, 4(%esp)    #,
        movl    %ebx, 8(%esp)    #,
        movl    (%ecx), %ebx     # argc, argc
        call    ___main  #
        movl    %ebx, (%esp)     # argc,
        call    _foo     #
        movl    4(%esp), %ecx    #,
        movl    8(%esp), %ebx    #,
        addl    $12, %esp        #,
        leal    -4(%ecx), %esp   #,
        ret

It doesn't seem to matter whether FOOPTR is const or static.  I also got
an ICE with -O2 -fwhole-program -fipa-pta:

out4.c: In function 'main':
out4.c:11: error: stmt (0x7fe086a0) marked modified after optimization
pass: 
FOOPTR.0_1 = FOOPTR;
out4.c:11: internal compiler error: verify_ssa failed
Please submit a full bug report,
with preprocessed source if appropriate.
See <URL:http://gcc.gnu.org/bugs.html> for instructions.

This is with mainline from about two months ago, I need to rebuild it to
see if it's still happens.

Brian