http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60452
--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #3)
> If I modify the shorter testcase such that int e[2]; is a static array, then
> we don't generate cmove for it, because on:
> (mem:SI (const:DI (plus:DI (symbol_ref:DI ("e") <var_decl 0x7ffff19d6098 e>)
> (const_int 524288 [0x80000]))) [2 e+524288 S4 A32])
> may_trap_or_fault_p returns true (correctly). But in the case of automatic
> out-of-bound access we get instead:
> (mem/c:SI (plus:DI (reg/f:DI 20 frame)
> (const_int 524272 [0x7fff0])) [2 S4 A128])
> and there may_trap_or_fault_p really can't (easily) know if it is valid or
> not, there is no MEM_EXPR even. This is because in the #c2 testcase as well
> as the original one DECL_RTL of e variable is a register, not MEM.
> But even if I try:
> int a;
> __attribute__((noinline, noclone)) void
> foo (int *e)
> {
> asm volatile ("" : : "r" (e) : "memory");
> }
>
> int
> main ()
> {
> int e[2] = { 0, 0 }, f = 0;
> if (a == 131072)
> f = e[a];
> foo (e);
> return f;
> }
> where we have:
> (mem:SI (plus:DI (reg/f:DI 20 frame)
> (const_int 524272 [0x7fff0])) [2 e+524288 S4 A128])
> instead and thus from MEM_EXPR we perhaps could find out that it is an out
> of bound access, we still always treat all frame based accesses (whatever
> the offset is) as non-trapping.
> So perhaps we need to handle known out of bound MEMs specially when we find
> that fact out (if we want to emit them into the RTL IL at all), one thing is
> expansion, another thing if say initially non-constant offset is later
> CSEd/forwprop etc. into constant out of bound offset.
>
> Thoughts?
Well, may_trap_or_fault_p has to be conservative. See how tree_could_trap_p
handles bound violations. If you can't prove it isn't out-of-bounds
then you have to assume it may be.
