Clang's call lowering often coerces pointers to ints, which blocks alias analysis

[Quuxplusone]

Bugzilla Link	PR31724
Status	NEW
Importance	P enhancement
Reported by	Reid Kleckner (rnk@google.com)
Reported on	2017-01-23 12:04:20 -0800
Last modified on	2017-01-23 12:33:04 -0800
Version	unspecified
Hardware	PC Windows NT
CC	dberlin@dberlin.org, llvm-bugs@lists.llvm.org
Fixed by commit(s)
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Consider this C code:

struct Handle { void *p; };
extern void g(struct Handle);
void f(struct Handle o) { g(o); }

Here are three examples of clang coercing the 'p' field to an int:

$ clang -S -x c --target=ppc64-linux t.cpp  -o - -emit-llvm  | grep define
define void @f(i64 %o.coerce) #0 {

$ clang -S -x c --target=x86_64-windows t.cpp  -o - -emit-llvm  | grep define
define void @f(i64 %o.coerce) #0 {

$ clang -S -x c --target=mips-linux t.cpp  -o - -emit-llvm  | grep define
define void @f(i32 inreg %o.coerce) #0 {

According to dannyb, this is bad, because the ptrtoint calls block alias
analysis.

[Quuxplusone]

In general, we can't do anything with pointer to int'd pointers. We can't tell
where they point to.
In an IPA analysis, we can't easily join them up, etc.

We also assume they escape.

BasicAA only handles bitcasts, it doesn't go back through ptrtoint/inttoptr to
see if they are the same thing, unmodified (and doing so is expensive), so even
inlining won't help until the inttoptr/ptrtoint pair is eliminated.

There are a couple ways to solve this.

Truthfully, the best way is to make them pointers again if possible.

If that really can't be solved, we should add metadata saying they are special,
and really pointers, and include the real pointer address expression somewhere,
but this seems .. hard.

[Quuxplusone]

(and by special, i mean "only escape/etc as much as a pointer would, and not 'complete lost to the void'", which is usually how ptrtoint is treated)