[DAGCombine] Fold overlapping constant stores

[niravd]

Summary

Fold a smaller constant store into larger constant stores immediately
preceeding it.

[rnk]

llvm/include/llvm/CodeGen/SelectionDAGAddressAnalysis.h

67

Why zero initialize here but not in the other call site? It gives the impression that it reads it as an input, but it doesn't.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

15444

Implementing this FIXME is just a matter of changing the Offset * 8 calculation below, right? And, updating test cases, of course. If so, I think it's easy enough to be worth implementing just for completeness.

[niravd]

Add big endian support and rebase past added big and little endian test case.

[courbet]

LGTM

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

15440

nit: preceding (here and elsewhere)

15453

clang-format ?

[niravd]

LGTMed, but not accepted. Presumably oversight, but address nits and clang format just in case.

[lebedev.ri]

I *think* this likely caused https://bugs.llvm.org/show_bug.cgi?id=43446

[lebedev.ri]

In D58468#1731906, @lebedev.ri wrote:

I *think* this likely caused https://bugs.llvm.org/show_bug.cgi?id=43446

Reverted in rGd20907d1de89bf63b589fadd8c096d4895e47fba.

[lebedev.ri]

This review seems to be stuck/dead, consider abandoning if no longer relevant.