tree be9e251de60c91f7ee6487249ecb38a1c40e2083
parent f6f2b8d5fd975c795bf70d98a29566ca041bebd1
author Nikita Popov <nikita.ppv@gmail.com> 1544217418 +0000
committer Nikita Popov <nikita.ppv@gmail.com> 1544217418 +0000

[MemCpyOpt] memset->memcpy forwarding with undef tail

Currently memcpyopt optimizes cases like

    memset(a, byte, N);
    memcpy(b, a, M);

to

    memset(a, byte, N);
    memset(b, byte, M);

if M <= N. Often this allows further simplifications down the line,
which drop the first memset entirely.

This patch extends this optimization for the case where M > N, but we
know that the bytes a[N..M] are undef due to alloca/lifetime.start.

This situation arises relatively often for Rust code, because Rust does
not initialize trailing structure padding and loves to insert redundant
memcpys. This also fixes https://bugs.llvm.org/show_bug.cgi?id=39844.

For the implementation, I'm reusing a bit of code for a similar existing
optimization (direct memcpy of undef). I've also added memset support to
MemDepAnalysis GetLocation -- Instead, getPointerDependencyFrom could be
used, but it seems to make more sense to add this to GetLocation and thus
make the computation cachable.

Differential Revision: https://reviews.llvm.org/D55120

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348645 91177308-0d34-0410-b5e6-96231b3b80d8