[PATCH] [InstCombine] Lower unknown @llvm.objectsize early.

[Nick Lewycky]

Ahmed Bougacha wrote:
> We already try to lower objectsize in InstCombine, but only if the
> result is known.  When it is unknown, the intrinsic calls would survive
> the mid-level optimizers, to be lowered late, in CodeGenPrepare.
>
> We can lower them in InstCombine as well, to 0 or -1, depending on the
> min argument.
> One could argue this is a bit early to do the lowering, since the size
> might be made apparent by later optimizations.

Yep.

   In practice however,
> the majority of cases has a never-known objectsize, and in a lot of
> the remaining few, the size is immediately apparent (say from a
> global value, or an alloca).

Sure I'd expect that, but that isn't really a problem on its own.

> Always keeping the intrinsic call intact prevents optimizations, and
> makes memcpyopt useless when libcall fortification is enabled (the
> default on a few major targets).

Can we fix that instead? Not waiting as long as possible to lower 
unknown llvm.objectsize just can't be right.

> There are a few alternatives to doing it here; all seem overkill:
> - teach the libcall simplifier to look through unknown
>    @llvm.objectsize, so it's able to replace the fortified version
>    of a libcall with the non-fortified one if the size is unknown.
> - make a dedicated pass that lowers the intrinsic, even when
>    the size is unknown, run it before memcpyopt&  friends.
> - in memcpyopt, start by running the libcall simplifier, to
>    lower @llvm.objectsize, even when the size is unknown.

The first of those sounds best to me.

> Also, Nuno proposes a related improvement to the associated
> APIs (MemoryBuiltins.h): "[the analyzer] could be improved to
> produce an interval instead.  If we know that the minimum
> objectsize is larger than the written size, then the check could
> go away, even if we don't know the exact size of the buffer."
>
> http://reviews.llvm.org/D7129
>
> Files:
>    lib/Transforms/InstCombine/InstCombineCalls.cpp
>    test/Transforms/InstCombine/objsize.ll
>    test/Transforms/InstCombine/stpcpy_chk-1.ll
>    test/Transforms/InstCombine/strcpy_chk-1.ll
>
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