Re: PATCH RFA: PR 29286: Handle placement new aliasing issues

[Ian Lance Taylor <iant at google dot com>]

Eric Botcazou <ebotcazou@adacore.com> writes:

> > TYPE_REF_CAN_ALIAS_ALL disables the whole analysis.  That is, if P has
> > a type with TYPE_REF_CAN_ALIAS_ALL, then the alias analysis code
> > assumes that P could point to V, even if it can't see any way that the
> > address of V could have wound up in the variable P.
> 
> I don't think that's true.  TYPE_REF_CAN_ALIAS_ALL only disables the last step 
> too, albeit in a more heavyweight way.

Sorry, you're right, it only assumes that P could point to V if V is
call clobbered.  And it assumes that a cast to or from a
TYPE_REF_CAN_ALIAS_ALL pointer is an escape site, which has the effect
of call clobbering any variable to which any ALIAS_ALL pointer could
point.

But I believe I am still correct in saying that TYPE_REF_CAN_ALIAS_ALL
disables the whole analysis.  It replaces it with a much simpler
analysis: a TYPE_REF_CAN_ALIAS_ALL cast is like a function call, and a
TYPE_REF_CAN_ALIAS_ALL pointer P can point to any call clobbered
variable V.  Presumably setting a TYPE_REF_CAN_ALIAS_ALL pointer to
the address of a variable counts as a cast and thus counts as call
clobbering the variable.  This effectively disables all steps of the
alias analysis code with respect to TYPE_REF_CAN_ALIAS_ALL pointers,
and also disables much of the alias analysis applied to variables
whose address is taken by TYPE_REF_CAN_ALIAS_ALL pointers.  Or so it
seems to me.

> > I hope you have some test cases for it.
> 
> Yes, for example this one which now appears in the logs on mainline:
>   FAIL: gnat.dg/address_conversion.adb execution test

Alas, I don't understand what that test case is doing.

Ian