• d-bugmail puremagic.com (31/31) Jan 17 https://issues.dlang.org/show_bug.cgi?id=24343

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=24343

          Issue ID: 24343
           Summary: Read only data used to initialize objects that are
                    mostly zero
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: bugzilla digitalmars.com

Consider:

struct S { int[100] x; }

The compiler generates a static initializer for it. But since it is all zeros,
it is allocated in the BSS segment and does not consume any memory in the DATA
or READONLY sections in the executable. Even better, since the initializer is
readonly, all the zero-initialized objects sit on top of each other in BSS,
consuming only the memory used by the largest.

When a zero initializer object is initialized, it is done with a memset().

For:

struct T { int i = 3; int[100] x; }

The initializer is not all zero, so it is placed in the READONLY section.
Initialization of an object is performed by doing a memcpy() from this data.

The idea of this issue is:

1. the .init data not be generated at all for the T

2. Object initialization should be done by a memset() followed by assignment to
the non-zero members. Or simply do memberwise assignment.

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