Reviewers: Benedikt Meurer,
Message:
The CL is actually not as big as it seems, there is a lot of re-shuffling
of the
code going on, and platform ports are extremely similar now. :-)
Description:
Consistenly handle power-of-2 divisors in division-like operations
Lithium currently supports 3 division-like operations on integral operands:
"Normal" division (rounding towards zero), flooring division (rounding
towards
-Infinity) and modulus calculation (the counterpart for the "normal"
division).
For divisors which are a power of 2, one can efficiently use some bit
fiddling
to avoid the actual division for such operations. This CL cleanly splits off
these operations into separate Lithium instructions, making the code much
more
maintainable and more consistent across platforms.
There are 2 basic variations of these bit fiddling algorithms: One involving
branches and a seemingly more clever one without branches. Choosing between
the
two is not as easy as it seems: Benchmarks (and probably real-world)
programs
seem to favor positive dividends, registers and shifting units are sometimes
scarce resources, and branch prediction is quite good in modern processors.
Therefore only the "normal" division by a power of 2 is implemented in a
branch-free manner, this seems to be the best approach in practice. If this
turns out to be wrong, we can easily and locally change this.
Please review this at https://codereview.chromium.org/175143002/
SVN Base: https://v8.googlecode.com/svn/branches/bleeding_edge
Affected files (+1444, -940 lines):
M src/a64/lithium-a64.h
M src/a64/lithium-a64.cc
M src/a64/lithium-codegen-a64.cc
M src/arm/lithium-arm.h
M src/arm/lithium-arm.cc
M src/arm/lithium-codegen-arm.cc
M src/hydrogen-instructions.h
M src/ia32/lithium-codegen-ia32.cc
M src/ia32/lithium-ia32.h
M src/ia32/lithium-ia32.cc
M src/utils.h
M src/x64/lithium-codegen-x64.cc
M src/x64/lithium-x64.h
M src/x64/lithium-x64.cc
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