Hi,
On Sat, Nov 03, 2012 at 09:01:53AM +0000, Yangyueming wrote:
> Hi, all
>
> I do the research of min max instructions recently. I find it is related with
> phiopt.
>
> case1:
> int foo(short a ,short b)
> {
> if (a < b)
> a = b;
> return a;
> }
>
> It is successed in pass phiopt1(-O2 with gcc 4.7.0). The MAX_EXPR can be
> generated.
>
> foo (short int a, short int b)
> {
> int D.2094;
>
> <bb 2>:
> a_9 = MAX_EXPR <a_2(D), b_3(D)>;
> D.2094_5 = (int) a_9;
> return D.2094_5;
>
> }
>
> But when I do the test for a case with a little change, it is failed to
> generate MAX_EXPR in phiopt1.
> The failed case2 :
> int foo(short a ,short b)
> {
> int c;
> if (a < b)
> a = b;
>
> c = *(int *)&a;
ehm, and just what do you expect the result of the above line to be?
> return c;
> }
>
> I find it is because of the esra pass failed to generate PHI NODE.
First and foremost, esra only deals with aggregate types and nothing
in any of your examples is an aggregate, therefore esra does not touch
your examples in any way. Even though the a$0 temporary has a dollar
sign in it, that does not mean it is created by SRA. On my i686
desktop the temporary is called a.0 and it is generated because in
gimple all scalar loads and stores have to go through a variable in
SSA form.
Nevertheless, I can tell why there is no phi node for a. a has its
address taken and therefore it is not in SSA form (as clearly visible
in the first statements in bb 2 and bb 3). Thus, no PHI nodes for a.
> Dump of phifail.c.027t.esra:
> foo (short int a, short int b)
> {
> int c;
> short int a$0;
>
> <bb 2>:
> a$0_1 = a;
> if (a$0_1 < b_2(D))
> goto <bb 3>;
> else
> goto <bb 4>;
>
> <bb 3>:
> a = b_2(D);
>
> <bb 4>:
> c_4 = MEM[(int *)&a];
> c_5 = c_4;
> return c_5;
>
> }
>
> Why it is failed and if there's a way to make it work?
I believe that the modified input has undefined semantics (sorry, I
don't have time to verify this in the standard) and so no, there is no
way to make it work.
Martin
