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[MemCpyOpt] memset->memcpy forwarding with undef tail

Authored by nikic on Dec 7 2018, 1:16 PM.

Description

[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

llvm-svn: 348645