[Bug gdb/17143] New: default obstack alignment wastes a lot of space on amd64-linux

[Bug gdb/17143] New: default obstack alignment wastes a lot of space on amd64-linux

[dje at google dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17143

            Bug ID: 17143
           Summary: default obstack alignment wastes a lot of space on
                    amd64-linux
           Product: gdb
           Version: HEAD
            Status: NEW
          Keywords: performance
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: dje at google dot com

The default obstack alignment on amd64-linux is 16 (sizeof (long double)).
This wastes a fair bit of space.

In one standard benchmark I use (with fission + .gdb_index):

With 16 byte alignment:

(gdb) mt set per on
(gdb) file benchmark
...
Command execution time: 16.404199 (cpu), 16.541558 (wall)
Space used: 721887232 (+718557184 for this command)
#symtabs: 5695 (+5695), #primary symtabs: 86 (+86), #blocks: 1061 (+1061)

With 8 byte alignment:

(gdb) mt set per on
(gdb) file benchmark
...
Command execution time: 15.198143 (cpu), 15.235198 (wall)
Space used: 688427008 (+685096960 for this command)
#symtabs: 5695 (+5695), #primary symtabs: 86 (+86), #blocks: 1061 (+1061)

[The cpu time difference isn't always as great, but I am consistently seeing a
0.25s difference at best.]

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