Wrong value for sext i1 true in BasicAliasAnalysis

[Nicholas White]

Hm, this turns out to be quite a bit more complicated than I thought.
Consider the test IR below - different wrapping flags (nsw, nuw) on the add
and sub functions give different guarantees, so while %c MustAlias to %a,
%b and %d only PartialAlias to %a as if the arithmetic wraps then %b and %d
just NoAlias %a. I think GetLinearExpression needs to know a bit more about
modular arithmetic, and when it's safe to ignore extensions (e.g. a "zext"
of a value produced by "add nsw nuw" can be ignored). I've made a very
conservative patch here: http://reviews.llvm.org/D6682 that refuses to
process any BinaryOps if the result is going to be extended in some way
(the Extension == EK_NotExtended check) and have added the tests cases on
this thread to it. However, the patch causes "make check" to fail as it
makes the BasicAA pass returns PartialAlias results where the current tip
returns NoAlias. I can't find any library functions that do symbolic
modular arithmetic in the codebase (would ValueTracking be the logical
place for this?), so I'm going to work a bit more on the logic in
GetLinearExpression. Any ideas would be greatly appreciated :)

Thanks -

Nick

define void @test_path_dependence(i32 %p, i8* %mem) {
  %p.minus1 = add i32 %p, -1 ; this will always unsigned-wrap, unless %p ==
0
  %p.minus1.64 = zext i32 %p.minus1 to i64
  %p.64.again = add i64 %p.minus1.64, 1 ; either %p (if we wrapped) or
4294967296 (if we didn't)

  %p.nsw.nuw.minus1 = sub nsw nuw i32 %p, 1 ; as nuw we know %p >= 1
  %p.nsw.nuw.minus1.64 = zext i32 %p.nsw.nuw.minus1 to i64
  %p.nsw.nuw.64.again = add nsw nuw i64 %p.nsw.nuw.minus1.64, 1 ; ...so
always exactly %p

  %p.nsw.minus1 = sub nsw i32 %p, 1 ; only nsw, so can only guarantee %p !=
0x10000000
  %p.nsw.minus1.64 = zext i32 %p.nsw.minus1 to i64 ; when %p > 0x10000000
(ie <= 0 as a signed number)  then the zext will make this a huge positive
number
  %p.nsw.64.again = add nsw i64 %p.nsw.minus1.64, 1 ; ...and so this is
very much != %p

  %p.64 = zext i32 %p to i64
  %a = getelementptr inbounds i8* %mem, i64 %p.64
  %b = getelementptr inbounds i8* %mem, i64 %p.64.again
  %c = getelementptr inbounds i8* %mem, i64 %p.nsw.nuw.64.again
  %d = getelementptr inbounds i8* %mem, i64 %p.nsw.64.again
  ret void
}
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