Michael Kuperstein 92631b8db7 [BasicAA] Improve precision of alloca vs. inbounds GEP alias queries ... If a we have (a) a GEP and (b) a pointer based on an alloca, and the beginning of the object the GEP points would have a negative offset with repsect to the alloca, then the GEP can not alias pointer (b). For example, consider code like: struct { int f0, int f1, ...} foo; ... foo alloca; foo *random = bar(alloca); int *f0 = &alloca.f0 int *f1 = &random->f1; Which is lowered, approximately, to: %alloca = alloca %struct.foo %random = call %struct.foo* @random(%struct.foo* %alloca) %f0 = getelementptr inbounds %struct, %struct.foo* %alloca, i32 0, i32 0 %f1 = getelementptr inbounds %struct, %struct.foo* %random, i32 0, i32 1 Assume %f1 and %f0 alias. Then %f1 would point into the object allocated by %alloca. Since the %f1 GEP is inbounds, that means %random must also point into the same object. But since %f0 points to the beginning of %alloca, the highest %f1 can be is (%alloca + 3). This means %random can not be higher than (%alloca - 1), and so is not inbounds, a contradiction. Differential Revision: http://reviews.llvm.org/D20495 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270777 91177308-0d34-0410-b5e6-96231b3b80d8		2016-05-25 22:23:08 +00:00
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llvm	[BasicAA] Improve precision of alloca vs. inbounds GEP alias queries	2016-05-25 22:23:08 +00:00
llvm-c	Extract renaming from D19181	2016-05-23 16:38:25 +00:00