[llvm-dev] [RFC] Adding a -memeq-lib-function flag to allow the user to specify a memeq function.

David Jones via llvm-dev llvm-dev at lists.llvm.org
Mon Jan 7 04:43:22 PST 2019

On Mon, Jan 7, 2019 at 5:50 AM Clement Courbet <courbet at google.com> wrote:

> Hi David & James,
> On Sat, Jan 5, 2019 at 1:17 AM David Jones <david.jones at metrics.ca> wrote:
>> If we are considering an optimization to convert calls to memcmp into
>> bcmp, then does it make sense to add an intrinsic for bcmp like there is
>> for memcmp?  That way IR writers can express their requirements precisely:
>> memcmp if you care about the direction of inequality, and bcmp if you do
>> not.
> As mentioned in my answer to Hal above, I think the backend at least
> should be able to do this.
> Then adding an intrinsic is only for for convenience, because if the
> backend can do the optimization automatically it's enough for the frontend
> to emit the attribute. For example, for a language that has a runtime which
> as a memeq/bcmp, the frontend could just emit the call site annotation. I
> have no strong opinion on whether the convenience justifies an intrinsic.
However, there may be cases where the optimizer cannot determine that a
call to memcmp can be reduced to bcmp.  e.g. I pass the result of memcmp()
to some externally-defined function unknown to LLVM. I would like to be
able to specify bcmp explicitly in this case.
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