cmd/go: cache compiler flag info
When you run 'go env' or any command that needs to consider
what the default gcc flags are (such as 'go list net' or
'go list <any package with net as a dependency>'),
the go command runs gcc (or clang) a few times to see what
flags are available.
These runs can be quite expensive on some systems, particularly
Macs that seem to need to occasionally cache something before
gcc/clang can execute quickly.
To fix this, cache the derived information about gcc under a cache
key derived from the size and modification time of the compiler binary.
This is not foolproof, but it should be good enough.
% go install cmd/go
% time go env >/dev/null
0.22 real 0.01 user 0.01 sys
% time go env >/dev/null
0.03 real 0.01 user 0.01 sys
%
Fixes #50982.
Change-Id: Iba7955dd10f610f2793e1accbd2d06922f928faa
Reviewed-on: https://go-review.googlesource.com/c/go/+/392454
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>