# [SelectionDAG][RFC] Allow the user to specify a memeq function (v3).AbandonedPublicActions

Authored by courbet on Jan 4 2019, 4:45 AM.

# Details

Reviewers
None
Summary

Right now, when we encounter a string equality check,
e.g. if (memcmp(a, b, s) == 0), we try to expand to a comparison if s is a
small compile-time constant, and fall back on calling memcmp() else.

This is sub-optimal because memcmp has to compute much more than
equality.

This patch adds a way for the user to specify a memeq library
function using a function attribute, e.g.:

```attributes #0 = { "memeq-func-name"="user_memeq" }

...

%m = tail call i32 @memcmp(i8* %a, i8* %b, i64 %s) nounwind #0
%c = icmp eq i32 %m, 0
ret i1 %c```

This function will be called instead of memcmp() when the result of the memcmp
call is only used for equality comparison.

memeq can be made much more efficient than memcmp because equality
compare is trivially parallel while lexicographic ordering has a chain
dependency.

# Diff Detail

Repository
rL LLVM
Build Status
 Buildable 26395 Build 26394: arc lint + arc unit

### Event Timeline

courbet created this revision.Jan 4 2019, 4:45 AM
courbet updated this revision to Diff 180225.Jan 4 2019, 4:47 AM

Fix comment.

courbet abandoned this revision.Mar 12 2019, 12:26 AM
Herald added a project: Restricted Project. Mar 12 2019, 12:26 AM