Reviewers: Weiliang,
Message:
PTAL
Description:
X87: Move hash code from hidden string to a private symbol
port eca5b5d7abc0a9028cb9832087fbf2ed59dadf92 (r28622).
original commit message:
* Hash code is now just done with a private own symbol instead of the
hidden
string, which predates symbols.
* In the long run we should do all hidden properties this way and get
rid of
the
hidden magic 0-length string with the zero hash code. The advantages
include
less complexity and being able to do things from JS in a natural way.
* Initially, the performance of weak set regressed, because it's a
little
harder
to do the lookup in C++. Instead of heroics in C++ to make things
faster I
moved some functionality into JS and got the performance back. JS is
supposed to be good at looking up named properties on objects.
* This also changes hash codes of Smis so that they are always Smis.
Performance figures are in the comments to the code review. Summary:
Most
of js-perf-test/Collections is neutral. Set and Map with object keys are
40-50%
better. WeakMap is -5% and WeakSet is +9%.
In the code review comments is a patch with an example of the heroics we
could do in C++ to make lookup faster (I hope we don't have to do this.
Instead
of checking for the property, then doing a new
In a similar vein we could give the magic zero hash code to the hash
code
symbol. Then when we look up the hash code we would sometimes see the
table
with all the hidden properties. This dual use of the field for either
the
hash
code or the table with all hidden properties and the hash code is rather
ugly,
and this CL gets rid of it. I'd be loath to bring it back. On the
benchmarks quoted above it's slightly slower than moving the hash code
lookup to
JS like in this CL.
One worry is that the benchmark results above are more monomorphic than
real
world code, so may be overstating the performance benefits of moving to
JS.
I
think this is part of a general issue we have with handling polymorphic
code
in
JS and any solutions there will benefit this solution, which boils down
to
regular property access. Any improvement there will lift all boats.
BUG=
Please review this at https://codereview.chromium.org/1153963010/
Base URL: https://chromium.googlesource.com/v8/v8.git@master
Affected files (+1, -0 lines):
M src/x87/macro-assembler-x87.cc
Index: src/x87/macro-assembler-x87.cc
diff --git a/src/x87/macro-assembler-x87.cc b/src/x87/macro-assembler-x87.cc
index
487790d9b6ac4e9546fb64f38a1a80e4ff05135b..4d1079f020e5e50205cf594288a433465317baed
100644
--- a/src/x87/macro-assembler-x87.cc
+++ b/src/x87/macro-assembler-x87.cc
@@ -1114,6 +1114,7 @@ void MacroAssembler::GetNumberHash(Register r0,
Register scratch) {
mov(scratch, r0);
shr(scratch, 16);
xor_(r0, scratch);
+ and_(r0, 0x3fffffff);
}
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