114728 - Our code could be smaller without exceptions and RTTI

Status: RESOLVED INVALID

(SeaMonkey :: Build Config, defect)

Description

[Conrad Carlen (not reading bugmail)]

Currently, both of these options are on in IDE_Options.h
size of xpcom.shlb with these on: 1,108,840 bytes
size of xpcom.shlb with these off: 1,013,732 bytes

This could be some low hanging fruit.

Comment 1

[Mike Pinkerton (not reading bugmail)]

it's already off on the mach-o build. my only concern is that if we turn rtti
off that it will cause problems for apps that have it on (embedding apps).
Doesn't it change the ABI?

Comment 2

[Conrad Carlen (not reading bugmail)]

scc - Is it possible for an embedding app built with RTTI to use our code built
without?

Comment 3

[Simon Fraser [no longer active]]

See bug 73606.

Comment 4

[Conrad Carlen (not reading bugmail)]

Yes, we do need to find out whether RTTI changes the ABI.

As far as bug 73606, compiling nsStdLib without exceptions is the least of its
problems. The big problem with nsStdLib is that it implements
void* operator new(size_t size); as if it were
void* operator new(size_t size, const nothrow_t&);
That, whether compiled with exceptions on or off, turns them off where embeddors
using nsStdLib depend on them most. 

Comment 5

[Conrad Carlen (not reading bugmail)]

After looking at MWRTTI.cp and some disassembly, objects and their vtables
compiled with RTTI can co-exist. The first entry in a CW vtable is a pointer to
an RTTI info struct. If compiled without RTTI, this pointer is still in the
vtable, it's just always zero. The offsets the the member functions remain the
same. Here's this class:

class foo {
public:
    foo();
    virtual ~foo();
    
    virtual void init();
};

Here's its disassembled vtable with RTTI on:

Hunk:
Kind=HUNK_GLOBAL_IDATA   Align=4  Class=RW  Name="__vt__3foo"(13)  Size=16
00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 '................'
XRef:
Kind=HUNK_XREF_32BIT     Offset=$00000000  Class=RW  Name="__RTTI__3foo"(154)
XRef:
Kind=HUNK_XREF_32BIT     Offset=$0000000C  Class=DS  Name="init__3fooFv"(155)
XRef:
Kind=HUNK_XREF_32BIT     Offset=$00000008  Class=DS  Name="__dt__3fooFv"(156)

and with RTTI off:

Hunk:
Kind=HUNK_GLOBAL_IDATA   Align=4  Class=RW  Name="__vt__3foo"(13)  Size=16
00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 '................'
XRef:
Kind=HUNK_XREF_32BIT     Offset=$0000000C  Class=DS  Name="init__3fooFv"(153)
XRef:
Kind=HUNK_XREF_32BIT     Offset=$00000008  Class=DS  Name="__dt__3fooFv"(154)

Notice that the size is the same, and the offsets are the same. They both have
room for the RTTI info, it'll just never be filled in on the one compiled
without RTTI.

Comment 6

[Conrad Carlen (not reading bugmail)]

Mass move to 1.0.1

Comment 7

[Conrad Carlen (not reading bugmail)]

Not an issue since we're not using this build environment going forward.