optimization/8967: Making class data members `const' pessimizes code

[martin at xemacs dot org]

>Number:         8967
>Category:       optimization
>Synopsis:       Making class data members `const' pessimizes code
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Mon Dec 16 15:16:00 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     martin@xemacs.org
>Release:        gcc-3.2.1
>Organization:
>Environment:
Linux x86
>Description:
Adding the `const' attribute to a C++ class data member
should never cause worse code to be emitted.  But this
is what happens in the example below.

g++ can optimize away the creation of unused temporaries,
but only if the temporaries' data members are mutable!

(See also PRs: 8952, 8936)

Source file:
-------------------------------------------------------
class Const
{
private: const int x, y;
public:
  Const (int X, int Y) : x (X), y (Y) {}
  inline friend Const operator+ (const Const& z1, const Const& z2)
  { return Const (z1.x + z2.x, z1.y + z2.y); }
};

class Mutable
{
private: int x, y;
public:
  Mutable (int X, int Y) : x (X), y (Y) {}
  inline friend Mutable operator+ (const Mutable& z1, const Mutable& z2)
  { return Mutable (z1.x + z2.x, z1.y + z2.y); }
};

Const Const_foo () { return Const(1,2) + Const(8,9); }
Mutable Mutable_foo () { return Mutable(1,2) + Mutable(8,9); }
-------------------------------------------------------
x86 asm for Const_foo:
	pushl	%ebp
	movl	%esp, %ebp
	subl	$16, %esp
	movl	8(%ebp), %eax
	movl	$1, -8(%ebp)
	movl	$8, -16(%ebp)
	movl	$2, -4(%ebp)
	movl	$9, -12(%ebp)
	movl	$9, (%eax)
	movl	$11, 4(%eax)
	movl	%ebp, %esp
	popl	%ebp
	ret	$4

x86 asm for Mutable_foo():
	pushl	%ebp
	movl	%esp, %ebp
	subl	$16, %esp
	movl	8(%ebp), %eax
	movl	$9, (%eax)
	movl	$11, 4(%eax)
	movl	%ebp, %esp
	popl	%ebp
	ret	$4

The functions have identical code, except for the
(useless) creation of the operands to the `+' operator.
>How-To-Repeat:
On Linux x86, run g++ -O3 -S and examine .s file.
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: