cmd/compile: avoid heap-allocating LocalSlot in addNamedValue

[siutsin]

addNamedValue takes &loc of a local ssa.LocalSlot and stores it in the long-lived f.Names
and f.CanonicalLocalSlots. Escape analysis is flow-insensitive, so this heap-allocates loc on
every call even when the slot already exists. Allocating only on the first-sighting path (mirroring
(*Func).localSlotAddr) keeps loc on the stack otherwise. Generated code is byte-identical.

compilebench -alloc, allocs/op, linux/amd64 (-count 20):

                         │ /tmp/old.txt │            /tmp/new.txt            │
                         │  allocs/op   │  allocs/op   vs base               │
Template                    820.9k ± 0%   816.2k ± 0%  -0.58% (p=0.000 n=20)
Unicode                     574.2k ± 0%   573.7k ± 0%  -0.07% (p=0.000 n=20)
GoTypes                     5.013M ± 0%   4.971M ± 0%  -0.83% (p=0.000 n=20)
Compiler                    702.2k ± 0%   698.6k ± 0%  -0.51% (p=0.000 n=20)
SSA                         47.02M ± 0%   46.50M ± 0%  -1.12% (p=0.000 n=20)
Flate                       823.9k ± 0%   813.8k ± 0%  -1.23% (p=0.000 n=20)
GoParser                    791.3k ± 0%   788.1k ± 0%  -0.41% (p=0.000 n=20)
Reflect                     2.318M ± 0%   2.302M ± 0%  -0.69% (p=0.000 n=20)
Tar                         937.7k ± 0%   930.1k ± 0%  -0.81% (p=0.000 n=20)
XML                         1.099M ± 0%   1.090M ± 0%  -0.77% (p=0.000 n=20)
LinkCompiler                729.0k ± 0%   729.1k ± 0%       ~ (p=0.101 n=20)
ExternalLinkCompiler        731.6k ± 0%   731.6k ± 0%       ~ (p=0.449 n=20)
LinkWithoutDebugCompiler    143.3k ± 0%   143.3k ± 0%       ~ (p=0.358 n=20)
geomean                     1.187M        1.181M       -0.54%

A CL follows.

cc @golang/compiler

[gopherbot]

Change https://go.dev/cl/790300 mentions this issue: cmd/compile: avoid heap-allocating LocalSlot in addNamedValue

[gabyhelp]

Related Code Changes

• cmd/compile: allocate less in regalloc's liveValues
• cmd/compile: avoid allocating a Nodes for readonly method receivers
• cmd/compile: reuse liveness structures
• cmd/compile: walk the progs to generate debug_lines
• cmd/compile: walk the progs to generate debug_lines
• cmd/compile: stack-allocate values worklist in schedule
• cmd/internal/obj: change the R field of LSym to []*Reloc
• cmd/compile/internal/ssa: reduce location list memory use
• [dev.debug] cmd/compile: track variable decomposition in LocalSlot
• cmd/compile: use desired info when allocating registers for live values

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[gopherbot]

Change https://go.dev/cl/790920 mentions this issue: cmd/compile: use a value slice for SSA Func.Names