Created Note Rust performance fixes (markdown)

Note-Rust-performance-fixes.md

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+This is a page of random performance ideas, mostly from pcwalton.
+
+pcwalton:
+graydon: there are a bunch of unnecessary allocations in trans
+pcwalton:
+I fixed a bunch of 'em but there are certainly more
+pcwalton:
+one thing that would very much help is a SmallVector in libstd
+
+pcwalton:
+fail() really needs to be a lang_item and it needs to be parameterized over ToStr
+pcwalton:
+the idea being that if you're failing with a constant string
+pcwalton:
+it should not call upcall_new_str_uniq() at the site of failure
+pcwalton:
+rather it should pass a string slice (&static/str) to a function which itself copies the string
+pcwalton:
+move the copy out of line
+pcwalton:
+likewise fmt! generates a ton of code. most of the time the unique string it produces is not really being used, except to be written once and thrown away
+graydon:
+mhm
+
+pcwalton:
+(1) stop zeroing stuff out to revoke cleanups, use a flag instead to indicate whether the dtor needs to be run (this is potentially a quite large win)
+pcwalton:
+(2) try to coalesce failure landing pads instead of duplicating the cleanups all the time
+
+pcwalton:
+also ty::sty comparison in the ty hashtable is calling into the shape glue. I'm going to try to fix that this week
+
+## Memory moves
+
+If you see large sequences of "mov" instructions, use `call_memmove` or `memzero` in trans, as appropriate. This is usually a symptom of code like `Store(bcx, Load(bcx, foo, bar), baz);`, which is *not* an efficient way to move large structural types around.