🚀 go-pugleaf

Hello,
	Most of the RISC architectures have a limited number
	bits in the field holding immediat operand. When the
	constant is too wide, it is not possible to use this
	field; so the compiler has to generate few instruc-
	tions (2 on a 32-bit machine) to load the constant 
	into a register.
	The problem with gcc is that transformation of an
	immediat operand into a register load is performed
	by the last pass (instruction selection). If this
	transformation takes place inside a loop, the load 
	could be move outside of the loop, but because the
	strength-reduction optimization has already been
	done, it is too late for that.

	----------------------------------------------------
		foo(int a[], int b[], int n)
		{
			int i;

			for (i = 0; i < n; i++)
			{
				a[i] = b[i] + 5000;
			}
		}
	----------------------------------------------------
					example 1

	The code below has been generated by gcc-1.40 with
	"-O -fstrength-reduce" on a Sparc station for the 
	example 1:
	----------------------------------------------------
				gcc_compiled.:
				.text
					.align 4
				.global _foo
					.proc 1
				_foo:
					!#PROLOGUE# 0
					save %sp,-112,%sp
					!#PROLOGUE# 1
					mov %i0,%o0
					mov %i1,%o2
					mov %i2,%o4
					mov 0,%o1
					cmp %o1,%o4
					bge L6
					nop
					mov %o0,%o3
				L5:
					ld [%o2],%o0
					sethi %hi(5000),%g1
					or %lo(5000),%g1,%g1
					add %o0,%g1,%o0
					st %o0,[%o3]
					add %o3,4,%o3
					add %o2,4,%o2
					add %o1,1,%o1
					cmp %o1,%o4
					bl L5
					nop
				L6:
					ret
					restore
	----------------------------------------------------

	The code below has been generated by acc, (the Sun
	ansi compiler) for the same example. You can notice
	that, this time, the register load, is outside of the
	loop.

	----------------------------------------------------
					.seg	"text"			! [internal]
				!.file	"foo.c"
				!ident	"acomp: (CDS) SunOS5.0 IDR3.5 (H6.1) 11/01/90"
					.proc	16
					.align	4
					.global	_foo
				_foo:
					mov	0,%o5
					cmp	%o5,%o2
					sethi	%hi(0x1388),%o3
					bge	LE12
					add	%o3,%lo(0x1388),%o4
				LY1:					! [internal]
					ld	[%o1],%o3
					inc	%o5
					add	%o3,%o4,%o3
					cmp	%o5,%o2
					inc	4,%o1
					st	%o3,[%o0]
					bl	LY1
					inc	4,%o0
				LE12:
					retl
					nop				! [internal]
	----------------------------------------------------

	So, is anybody is able to suggest a way to do the
	same with gcc, I will be pleased to know it.  Thanx.

Bye.

Serge Adda - A.C.R.I. Compiler Team
email: adda@acri.fr