This fixes a substantial performance issue when scrolling emails with
long/complicated contents, where scrolling down a single line can take
something like hundreds of ms before the screen is updated to reflect
the scroll. It's really bad if the email has lots of columns, e.g. like
if it's an html email that was passed through a filter (w3m, etc) to
render it.
Using pprof, I found that the multiple calls to vterm.ScreenCell.Attrs()
in styleFromCell were really really expensive. This patch replaces them
with a single call.
Here's a before and after with a simple, but very manual test of opening
a large email with contents that went through a w3m filter and
continuously scrolling up and down over and over for ~30 seconds:
*** Before:
----------------------------------------------------------+-------------
flat flat% sum% cum cum% calls calls% + context
----------------------------------------------------------+-------------
28.25s 100% | git.x2esr.x2eht..z2f..z7esircmpwn..z2faerc..z2fwidgets.PartSwitcher.Draw
0 0% 99.94% 28.25s 82.31% | git.x2esr.x2eht..z2f..z7esircmpwn..z2faerc..z2fwidgets.PartViewer.Draw
28.25s 100% | git.x2esr.x2eht..z2f..z7esircmpwn..z2faerc..z2fwidgets.Terminal.Draw
----------------------------------------------------------+-------------
28.25s 100% | git.x2esr.x2eht..z2f..z7esircmpwn..z2faerc..z2fwidgets.PartViewer.Draw
0 0% 99.94% 28.25s 82.31% | git.x2esr.x2eht..z2f..z7esircmpwn..z2faerc..z2fwidgets.Terminal.Draw
19.23s 68.07% | git.x2esr.x2eht..z2f..z7esircmpwn..z2faerc..z2fwidgets.Terminal.styleFromCell
6.04s 21.38% | github.x2ecom..z2fddevault..z2fgo..z2dlibvterm.Screen.GetCellAt
1.38s 4.88% | git.x2esr.x2eht..z2f..z7esircmpwn..z2faerc..z2flib..z2fui.Context.Printf
0.62s 2.19% | runtime.mapassign
0.43s 1.52% | runtime.mapaccess2
0.20s 0.71% | runtime.newobject
0.19s 0.67% | runtime.callers (inline)
0.07s 0.25% | runtime.makeslice
0.07s 0.25% | runtime.mallocgc
----------------------------------------------------------+-------------
19.23s 100% | git.x2esr.x2eht..z2f..z7esircmpwn..z2faerc..z2fwidgets.Terminal.Draw
0 0% 99.94% 19.23s 56.03% | git.x2esr.x2eht..z2f..z7esircmpwn..z2faerc..z2fwidgets.Terminal.styleFromCell
19.21s 99.90% | github.x2ecom..z2fddevault..z2fgo..z2dlibvterm.ScreenCell.Attrs
*** After:
----------------------------------------------------------+-------------
flat flat% sum% cum cum% calls calls% + context
----------------------------------------------------------+-------------
0.31s 100% | git.x2esr.x2eht/~sircmpwn/aerc/widgets.Terminal.Draw
0 0% 99.87% 0.31s 1.33% | github.x2ecom/ddevault/go-libvterm.NewPos
0.25s 80.65% | runtime.callers (inline)
0.04s 12.90% | runtime.gomcache (inline)
----------------------------------------------------------+-------------
8.40s 100% | github.x2ecom/ddevault/go-libvterm.Screen.GetCellAt
0 0% 99.87% 8.40s 36.11% | github.x2ecom/ddevault/go-libvterm.Screen.GetCell
7.14s 85.00% | github.x2ecom/ddevault/go-libvterm._cgoCheckPointer
0.54s 6.43% | runtime.callers (inline)
0.35s 4.17% | runtime.exitsyscall
0.11s 1.31% | runtime.deferprocStack
0.07s 0.83% | doentersyscall
0.07s 0.83% | runtime.getg
0.05s 0.6% | runtime.casgstatus
0.03s 0.36% | _init
0.03s 0.36% | runtime.gomcache (inline)
----------------------------------------------------------+-------------
8.46s 100% | git.x2esr.x2eht/~sircmpwn/aerc/widgets.Terminal.Draw
0 0% 99.87% 8.46s 36.37% | github.x2ecom/ddevault/go-libvterm.Screen.GetCellAt
8.40s 99.29% | github.x2ecom/ddevault/go-libvterm.Screen.GetCell
0.06s 0.71% | runtime.callers (inline)
----------------------------------------------------------+-------------
0.31s 100% | git.x2esr.x2eht/~sircmpwn/aerc/widgets.Terminal.styleFromCell
0 0% 99.87% 0.31s 1.33% | github.x2ecom/ddevault/go-libvterm.ScreenCell.Attrs
)