[PATCH] D84821: [InstCombine] Don't transfer BitCastInst at InstCombine pass when the user of BitCast is lifetime marker

[Chuanqi Xu via Phabricator via llvm-commits]

ChuanqiXu added a comment.

In D84821#2183027 <https://reviews.llvm.org/D84821#2183027>, @nikic wrote:

> I don't think this is the right approach to the problem. Can you give some more information on which places in LLVM assume that only bitcasts feed into lifetime intrinsics?

>From the `CodeExtractor.cpp`, `CoroFrame.cpp` and `StackLifetime.cpp`, I find such assumptions. I also find that there are other methods to collect `lifetime marker` of an instruction. For example, in the `Inline Module`, it finds among all the users of the instruction I  to find which is used by `lifetime marker`. And in the `InstCombineLoadStore.cpp`, I find it collects  `lifetime marker` by calculates the users with kind `BitCast`, `GEP` and `AddrSpaceCastInst` (which means there are cases which are not covered by this patch). What I want to say here is that the style of collecting `lifetime marker` seems a little out-of-order.

In D84821#2183271 <https://reviews.llvm.org/D84821#2183271>, @lebedev.ri wrote:

> I'm not sure any such assumption being made is sane. Those passes should be fixed instead.

Yes. We can fix this problem by either fixing other passes or fixing this pass. We need find out that which  one is more natural. In my thought, when I want to collect `lifetime marker` as a newbie here, I will go to see how the `lifetime marker` is created. So I find `CreateLifetimeStart` and `CreateLifetimeEnd` in LLVM and `CreateCall ('lifetime_start' or `lifetime_end`')` in clang's `CodeGen` module. In all of the methods, the logic is to judge whether the type of the source value is `i8*`. If the type of the source value is `i8*`, the `Create*` methods will create a direct call to `lifetime`  intrinsics. Otherwise, the `Create*` methods will create a `BitCast` to convert the type of source value to `i8*`. So from the process, I think it is natural for code reader who is not so familiar with LLVM to think that he can collect `lifetime marker` by judge the instruction itself and the users who is `BitCastInst`.


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