[optimization] EGCS is too pessimistic

[Gabriel Dos Reis <Gabriel dot Dos-Reis at dptmaths dot ens-cachan dot fr>]

Hi!

Consider (once again) the following code:

class array {
public:
    array(int n) : data(new double[n]), sz(n) {}

    double operator[](int i) const { return data[i]; }	// certainly a read
    double& operator()(int i) { return data[i]; }	// a write

private:
    double* const data;
    const int sz;
};


const int N = 20;

int main()
{
    array a(N), b(N), c(N);

    for (int i=0; i<N; ++i)
        a(i) = b[i] + c[i];
}


The members array::data and array::sz are *const* data members (any
construct intended to change their values results in an undefined
behaviour), so remain invariant throughout the main function,
especialy they are loop-invriants in the for-loop. Alas, EGCS
(egcs-2.92.18) is still pessimistic as ever and does generate loads
for them. Below is the (SPARC) assembly output on optimization level
-O4. 

I consider these optimizations necessary for the standard library
valarray component support.

-- Gaby


	.file	"b.C"
gcc2_compiled.:
.section	".text"
	.align 4
	.global main
	.type	 main,#function
	.proc	04
main:
.LLFB1:
	!#PROLOGUE# 0
	save	%sp, -136, %sp
.LLCFI0:
	!#PROLOGUE# 1
	add	%fp, -24, %l2
	call	__builtin_vec_new, 0
	mov	160, %o0
	st	%o0, [%fp-24]
	mov	20, %l0
	st	%l0, [%l2+4]
	add	%fp, -32, %l1
	call	__builtin_vec_new, 0
	mov	160, %o0
	st	%o0, [%fp-32]
	add	%fp, -40, %l2
	st	%l0, [%l1+4]
	call	__builtin_vec_new, 0
	mov	160, %o0
	st	%o0, [%fp-40]
	st	%l0, [%l2+4]
	mov	0, %o3
.LL15:
	sll	%o3, 3, %o2
	ld	[%fp-32], %o0		! unnecessary load
	ldd	[%o0+%o2], %f2
	ld	[%fp-40], %o1		! unnecessary load
	ldd	[%o1+%o2], %f6
	fmovs	%f2, %f4
	fmovs	%f3, %f5
	fmovs	%f6, %f2
	fmovs	%f7, %f3
	faddd	%f4, %f2, %f4
	ld	[%fp-24], %o0		! unncessary load
	add	%o3, 1, %o3
	cmp	%o3, 19
	ble	.LL15
	std	%f4, [%o0+%o2]
	ret
	restore %g0, 0, %o0
.LLFE1:
.LLfe1:
	.size	 main,.LLfe1-main

.section	".eh_frame",#alloc,#write
__FRAME_BEGIN__:
	.uaword	.LLECIE1-.LLSCIE1
.LLSCIE1:
	.uaword	0x0
	.byte	0x1
	.byte	0x0
	.byte	0x1
	.byte	0x7c
	.byte	0x65
	.byte	0xc
	.byte	0xe
	.byte	0x0
	.byte	0x9
	.byte	0x65
	.byte	0xf
	.align 4
.LLECIE1:
	.uaword	.LLEFDE1-.LLSFDE1
.LLSFDE1:
	.uaword	.LLSFDE1-__FRAME_BEGIN__
	.uaword	.LLFB1
	.uaword	.LLFE1-.LLFB1
	.byte	0x4
	.uaword	.LLCFI0-.LLFB1
	.byte	0xd
	.byte	0x1e
	.byte	0x2d
	.byte	0x9
	.byte	0x65
	.byte	0x1f
	.align 4
.LLEFDE1:
	.ident	"GCC: (GNU) egcs-2.92.18 19981103 (gcc2 ss-980609 experimental)"