[PATCH] run -Wnonnull later (PR 87489)

[Franz Sirl]

Am 2021-02-01 um 01:31 schrieb Martin Sebor via Gcc-patches:
> The initial -Wnonnull implementation in the middle end took place
> too late in the pipeline (just before expansion), and as a result
> was prone to false positives (bug 78817).  In an attempt to avoid
> the worst of those, the warning was moved to the ccp2 pass in
> r243874.  However, as the test case in PR 87489 shows, this is
> in turn too early and causes other false positives as a result.
> 
> A few experiments with running the warning later suggest that
> just before the mergephi2 pass is a good point to avoid this class
> of false positives without causing any regressions or introducing
> any new warnings either in Glibc or in Binutils/GDB.
> 
> Since PR 87489 is a GCC 8-11 regression I propose to make this
> change on the trunk as well as on the release branches.

Hi Martin,

I tested your patch and it showed also one more warning for this 
testcase with -O2 -Wnonnull:

class b {
public:
   long c();
};
class B {
public:
   B() : f() {}
   b *f;
};
long d, e;
class g : B {
public:
   void h() {
     long a = f->c();
     d = e = a;
   }
};
class j {
   j();
   g i;
};
j::j() { i.h(); }

I hope cvise didn't minimize too much, but at least in the original much 
larger code the warning looks reasonable too.

Franz