https://bugs.freedesktop.org/show_bug.cgi?id=85467
--- Comment #1 from Vinson Lee <v...@freedesktop.org> ---
This is introduced with llvm-3.6.0svn r220138.
9b2d091a9c2509a766a233a64453462faa2bdffc is the first bad commit
commit 9b2d091a9c2509a766a233a64453462faa2bdffc
Author: Chandler Carruth <chandl...@gmail.com>
Date: Sat Oct 18 06:36:22 2014 +0000
[InstCombine] Do an about-face on how LLVM canonicalizes (cast (load
...)) and (load (cast ...)): canonicalize toward the former.
Historically, we've tried to load using the type of the *pointer*, and
tried to match that type as closely as possible removing as many pointer
casts as we could and trading them for bitcasts of the loaded value.
This is deeply and fundamentally wrong.
Repeat after me: memory does not have a type! This was a hard lesson for
me to learn working on SROA.
There is only one thing that should actually drive the type used for
a pointer, and that is the type which we need to use to load from that
pointer. Matching up pointer types to the loaded value types is very
useful because it minimizes the physical size of the IR required for
no-op casts. Similarly, the only thing that should drive the type used
for a loaded value is *how that value is used*! Again, this minimizes
casts. And in fact, the *only* thing motivating types in any part of
LLVM's IR are the types used by the operations in the IR. We should
match them as closely as possible.
I've ended up removing some tests here as they were testing bugs or
behavior that is no longer present. Mostly though, this is just cleanup
to let the tests continue to function as intended.
The only fallout I've found so far from this change was SROA and I have
fixed it to not be impeded by the different type of load. If you find
more places where this change causes optimizations not to fire, those
too are likely bugs where we are assuming that the type of pointers is
"significant" for optimization purposes.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220138
91177308-0d34-0410-b5e6-96231b3b80d8
:040000 040000 982ecfc2706221c9731e5e8f54346370d2fe6283
667a68ea436b53840beda4b034cceee0e87dc6eb M lib
:040000 040000 a6f61cd09e99c468c8fdb8dd6bda10060b4f751f
cad7ad5d09cf0f43b3f47e36050c833b2debc9fc M test
bisect run success
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