[Bug tree-optimization/51506] New: Function cloning misses constant struct

[peteraward+gcc at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51506

             Bug #: 51506
           Summary: Function cloning misses constant struct
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: peteraward+...@gmail.com


The actual problem I’m dealing with is with avr-gcc, so the goal is to achieve
a small code size. I’m trying to write my code like this:
lcd_init(lcd_t l, ...)
where the first parameter is passed a *constant* struct which contains the
memory addresses of each of the pins for the LCD. Thus, I want the compiler to
note that all calls have the same first argument, clone the function, and
propagate the constant.

However, it doesn’t seem to be working in practice.
In trying to build this test case, I found the compiler would just inline all
the functions, which defeats the point (in the actual code, the cost of
inlining is too high). So, I’ve added the noinline attribute, which I don’t
think should stop this optimisation, but apologies if it does.

Anyhow, here’s the testcase.
(using gcc version 4.6.2 (Debian 4.6.2-5), on 64-bit Linux)

$ cat test.c
typedef struct {
    int a;
    int b;
} dint;

__attribute__((noinline))
static int compute_int(int x, int var) {
    int y = 0;
    for (int i = 0; i < x; i++)
        y += i * x;
    return y + var;
}

__attribute__((noinline))
static int compute_dint(dint x, int var) {
    int z = x.a + x.b;
    int y = 0;
    for (int i = 0; i < z; i++)
        y += i * z;
    return y + var;
}

int main() {
    int rv;
    rv += compute_dint((dint) {6, 1}, 1);
    rv += compute_dint((dint) {6, 1}, 2);
    rv += compute_dint((dint) {6, 1}, 3);
    rv += compute_int(5, 1);
    rv += compute_int(5, 2);
    rv += compute_int(5, 3);
    return rv;
}
$ gcc -fdump-ipa-all -fipa-cp -fipa-cp-clone -Os -std=c99 test.c

Expected result:
both compute_int and compute_dint should be optimised to versions where "x" is
constant.

Actual reslut:
only compute_int is optimised.