Re: [boost] Re: Re: Zero Unnecessary Temporary Objects: A Revolution

From: Mat Marcus (mmarcus_at_[hidden])
Date: 2002-10-13 09:43:59

--On Sunday, October 13, 2002 8:52 AM -0400 David Abrahams 
<dave_at_[hidden]> wrote:

> Furthermore, if the function being called is not inlined,
> pass-by-const& can result in a whole lot less code being
> generated than if pass-by-value is used.

Its interesting that you mention this. I'm not sure if this is 
a useful data point,, but I recently ran across a case where 
the opposite was true. I don't think it change the discsussion 
much, and it is probably just be a compiler bug, but I thought 
i'd mention it jsut the same.

With the Metrowerks Macintosh CFM targeted compilers 
call-by-const& can sometimes result in worse code. The issue 
involves passing actaul constants to a function by value versus 
by const&. If, say, an integer constant is passed by value the 
compiler generates reasonable code (see line 0000000C: in the 
CallByVal disassembly below). But in the pass by const& case 
the compiler decides that it has to add an entry to for the 
constant kNumbeOner the TOC (think of it as global data area). 
So we pay for at least an extra TOC entry (space + 
initialization code). Perhaps the compiler is trying to be 
"friendly" to folks who might cast-away-const? In any case, we 
actually had to clean up a bunch of code that suffered from 
this bloat recently.

 - Mat

const int kNumberOne=1;

void ByVal(int i) {
        
};
void ByRef(const int& i) {	
};

void CallByVal()
{
        ByVal(kNumberOne);
}

/*
Hunk:	Kind=HUNK_GLOBAL_CODE
Align=16  Class=PR  Name=".CallByVal__Fv"(3)  Size=36
00000000: 7C0802A6  mflr       r0
00000004: 90010008  stw        r0,8(SP)
00000008: 9421FFC0  stwu       SP,-64(SP)
0000000C: 38600001  li         r3,1
00000010: 48000001  bl         .ByVal__Fi
00000014: 80010048  lwz        r0,72(SP)
00000018: 38210040  addi       SP,SP,64
0000001C: 7C0803A6  mtlr       r0
00000020: 4E800020  blr
*/

void CallByRef()
{
        ByRef(kNumberOne);
}

/*
Hunk:	Kind=HUNK_GLOBAL_CODE
Align=16  Class=PR  Name=".CallByRef__Fv"(6)  Size=36
00000000: 7C0802A6  mflr       r0
00000004: 90010008  stw        r0,8(SP)
00000008: 9421FFC0  stwu       SP,-64(SP)
0000000C: 38620000  addi       r3,RTOC,kNumberOne
00000010: 48000001  bl         .ByRef__FRCi
00000014: 80010048  lwz        r0,72(SP)
00000018: 38210040  addi       SP,SP,64
0000001C: 7C0803A6  mtlr       r0
00000020: 4E800020  blr
*/