this was a tree of hashtables, and a query recursed into the table for the immediate dominator ad infinitum
if the initial lookup failed. This led to really bad performance on tall, narrow CFGs.
We can instead replace it with what is conceptually a multimap of value numbers to leaders (actually
represented by a hashtable with a list of Value*'s as the value type), and then
determine which leader from that set to use very cheaply thanks to the DFS numberings maintained by
DominatorTree. Because there are typically few duplicates of a given value, this scan tends to be
quite fast. Additionally, we use a custom linked list and BumpPtr allocation to avoid any unnecessary
allocation in representing the value-side of the multimap.
This change brings with it a 15% (!) improvement in the total running time of GVN on 403.gcc, which I
think is pretty good considering that includes all the "real work" being done by MemDep as well.
The one downside to this approach is that we can no longer use GVN to perform simple conditional progation,
but that seems like an acceptable loss since we now have LVI and CorrelatedValuePropagation to pick up
the slack. If you see conditional propagation that's not happening, please file bugs against LVI or CVP.
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instruction. Any that may be expanded otherwise by MC lowering should
override this value. rdar://8683274
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refusing to optimize two memcpy's like this:
copy A <- B
copy C <- A
if it couldn't prove that noalias(B,C). We can eliminate
the copy by producing a memmove instead of memcpy.
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there is no need to check to see if the source and dest of a memcpy are noalias,
behavior is undefined if not.
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if it is passed as a byval argument. The byval argument will just be a
read, so it is safe to read from the original global instead. This allows
us to promote away the %agg.tmp alloca in PR8582
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sahf movl 344(%rdi),%r14d
we used to produce:
t.s:2:1: error: unexpected token in argument list
^
we now produce:
t.s:1:11: error: unexpected token in argument list
sahf movl 344(%rdi),%r14d
^
rdar://8581401
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and testing is easier. A good example is the unknown-location.ll test that
now can just look for ".loc 1 0 0". We also don't use a DW_LNE_set_address for
every address change anymore.
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memset; we may need it to decide between MOVAPS and MOVUPS
later. Adjust a test that was looking for wrong code.
PR 3866 / 8675131.
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Some of these maps may merge in the future, but for now it's convenient to have
a utility function for them.
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memoize the results. This improves compile time in code which highly complex
expressions which get queried many times.
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It is generally not sufficient to check if the starting offset is in range
of the maximum offset that can be efficiently used for the target.
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This makes it more clear that the symbol is an internal, compiler-generated
name and gives a little more description about its contents.
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It was mistakenly looking at the pointer type when checking for the size of
global variables. This is a partial fix for Radar 8673120.
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