[JSC] Use (x + x) instead of (x * 2) when possible
https://bugs.webkit.org/show_bug.cgi?id=148051
Patch by Benjamin Poulain <[email protected]> on 2015-08-16
Reviewed by Michael Saboff.
When multiplying a number by 2, JSC was loading a constant "2"
in register and multiplying it with the first number:
mov $0x4000000000000000, %rcx
movd %rcx, %xmm0
mulsd %xmm0, %xmm1
This is a problem for a few reasons.
1) "movd %rcx, %xmm0" only set half of XMM0. This instruction
has to wait for any preceding instruction on XMM0 to finish
before executing.
2) The load and transform itself is large and unecessary.
To fix that, I added a StrengthReductionPhase to transform
multiplications by 2 into a addition.
Unfortunately, that turned the code into:
movsd %xmm0 %xmm1
mulsd %xmm1 %xmm0
The reason is GenerationInfo::canReuse() was not accounting
for nodes using other nodes multiple times.
After fixing that too, we now have the multiplications by 2
done as:
addsd %xmm0 %xmm0
(JSC::DFG::GenerationInfo::useCount):
(JSC::DFG::GenerationInfo::canReuse): Deleted.
- dfg/DFGSpeculativeJIT.cpp:
(JSC::DFG::FPRTemporary::FPRTemporary):
(JSC::DFG::SpeculativeJIT::canReuse):
(JSC::DFG::GPRTemporary::GPRTemporary):
- dfg/DFGStrengthReductionPhase.cpp:
(JSC::DFG::StrengthReductionPhase::handleNode):
