1200407 - Use EffectiveAddress instead of AddI to reduce register pressure

Status: NEW

(Core :: JavaScript Engine: JIT, defect, P5)

Description

[Brian Hackett [Laid off!]]

Attachment: patch

When there is an add instruction where both inputs are live afterwards, or one input is live and the other is a constant, on x86/x64 the register allocator ends up needing to copy an input at some point because the add will clobber its output register.  This hits at a couple of hot sites in a zlib inflation benchmark.

The attached patch lowers these instructions using LEffectiveAddress instead of LAddI.  This reduces the total number of Ion instructions executed by 1% and the amount of spill code executed by 4%.  Unfortunately it seems to slow down the actual run time of the benchmark by 1-2%, so it seems better to shelf this for now until there's a better understanding of what's going on.