On Wed, Nov 13 2019, Feng Xue OS wrote:
> Thanks.
>
> And for this issue, we can add a new tracker as a followup task.
It's now PR 92497.
Thanks,
Martin
>
> Feng
>
> ________________________________________
> From: Jan Hubicka <hubi...@ucw.cz>
> Sent: Tuesday, November 12, 2019 8:34 PM
> To: Feng Xue OS
> Cc: Martin Jambor; gcc-patches@gcc.gnu.org
> Subject: Re: Ping: [PATCH V6] Extend IPA-CP to support
> arithmetically-computed value-passing on by-ref argument (PR ipa/91682)
>
>>
>> OK, thanks - this looks like very nice ipa-prop improvement.
> Also note that there is a long standing problem with inlining ipacp
> clones. This can be shown on the following example:
>
> struct a {int a;};
> static int foo (struct a a)
> {
> return a.a;
> }
> __attribute__ ((noinline))
> static int bar (struct a a)
> {
> return foo(a);
> }
> main()
> {
> struct a a={1};
> return bar (a);
> }
>
> Now if you compile it with -O2 -fno-early-inlining ipacp correctly
> determines constants:
>
> Estimating effects for bar/1.
> Estimating body: bar/1
> Known to be false:
> size:6 time:14.000000 nonspec time:14.000000
> - context independent values, size: 6, time_benefit: 0.000000
> Decided to specialize for all known contexts, code not going to grow.
> Setting dest_lattice to bottom, because type of param 0 of foo is NULL or
> unsuitable for bits propagation
>
> Estimating effects for foo/0.
> Estimating body: foo/0
> Known to be false: op0[offset: 0] changed
> size:3 time:2.000000 nonspec time:3.000000
> - context independent values, size: 3, time_benefit: 1.000000
> Decided to specialize for all known contexts, code not going to grow.
>
>
> Yet the intended tranformation to "return 1" does not happen:
>
> __attribute__((noinline))
> bar.constprop (struct a a)
> {
> int a$a;
>
> <bb 2> [local count: 1073741824]:
> a$a_5 = a.a;
> return a$a_5;
>
> }
>
>
>
> ;; Function main (main, funcdef_no=2, decl_uid=1937, cgraph_uid=3,
> symbol_order=2) (executed once)
>
> main ()
> {
> struct a a;
> int _3;
>
> <bb 2> [local count: 1073741824]:
> a.a = 1;
> _3 = bar.constprop (a); [tail call]
> a ={v} {CLOBBER};
> return _3;
>
> }
>
> The problem here is that foo get inlined into bar and we never apply
> ipcp transform on foo, so a.a never gets constant propagated.
>
> For value ranges this works since late passes are able to propagate
> constants from value ranges we attach to the default def SSA names. I
> think correct answer here is to do no subtitution in in ipa-prop.c
> transform function. Rather note the known values for late passes and
> let FRE do its job.
>
> Honza
