[PATCH] Add basic support for removal of load that are fed by a store of an aggregate

[Björn Steinbrink]

2015-01-02 22:36 GMT+01:00 Chandler Carruth <chandlerc at google.com>:
>
> On Fri, Jan 2, 2015 at 1:27 PM, Björn Steinbrink <bsteinbr at gmail.com> wrote:
>>
>> If we have a simple load through a GEP that is fed by a store of an
>> aggregate, we can use the GEP indices to walk the stored aggregate and
>> extract the appropriate value to replace the load.
>
>
> What's the motivation for this change?

The rust compiler hit a case where not having this optimization caused
a branch not to be removed. The corresponding issue for rustc is
https://github.com/rust-lang/rust/issues/20149

> Note that SROA replaces all stores of aggregates with scalar stores of the
> components specifically so that neither it nor GVN needs to cope with
> aggregate loads or stores.

SROA did the opposite thing in this case, replacing individual stores
insertvalues and a store of an FCA. I'm attaching the full output of
opt -print-after-all -O2 -S for the failing test case given in the
rust issue. The first SROA creates the FCA store, and later when the
call to `unwrap` is inlined the existing optimizations can't eliminate
the branch that comes with it, because of the FCA store.

Björn