ottoni
afe1b1a705
There is not a clear path forward to safely using MMX registers as
scratch storage. Doing so spoils the state of the legacy x87 FPU, and the
x64 ABI uses the x87 FPU to implement long double. Our options are to
either:
1. Prohibit use of long double in source code (or x87 instructions in
machine code). Given that we have a dynamically linked binary that
includes system libraries outside our control, open source that needs
to be patched, fbcode, etc., this could prove difficult.
2. Always execute an FPU resetting instruction before transitioning from
TC code to C++. For all I know, these instructions are cheap, but it
still seems like an unfortunate overhead to impose everywhere; since
we don't want to mark every helper with a "reset fpu" call, we'd
probably end up bloating callsites.
3. Admit this doesn't work.
In the absence of evidence this matters for perf, I lean towards 3.
HipHop VM for PHP
HipHop VM (HHVM) is a new open-source virtual machine designed for executing programs written in PHP. HHVM uses a just-in-time compilation approach to achieve superior performance while maintaining the flexibility that PHP developers are accustomed to. HipHop VM (and before it HPHPc) has realized >5x increase in throughput for Facebook compared with Zend PHP 5.2.
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