Small Device C Compiler suite

Hi,

When I OR or AND an xdata location with a number, for instance to set a
bit in a register, the compiler generates an extra instruction to move
the old xdata contents into r2.  E.g.:

;	src/myfile.c:305: MY_REG &=3D ~BIT0;
;	genAssign
;	genAnd
	mov	dptr,#_MY_REG
	movx	a,@dptr
	mov	r2,a
;	Peephole 248.b	optimized and to xdata
	anl	a,#0xFE
	movx	@dptr,a

r2 is not then used anywhere, so why does this happen, and is there
anything I can do to stop it happening?

Mary-Ann

Re: [Sdcc-user] Redundant assembler instruction From: Maarten Brock <sourceforge.brock@ds...> - 2007-11-07 12:02 Hi Mary-Ann, > r2 is not then used anywhere, so why does this happen, and is there > anything I can do to stop it happening? Try the latest snapshot version of SDCC. It should optimize this. But in general this can always happen somewhere in your code. The register was used during code generation as an intermediate and then a peephole optimization rule removed the read of it as the value is still in the accumulator. It is much harder to determine it will not be read somewhere in the future so it is safest to keep the write. HTH, Maarten

Re: [Sdcc-user] Redundant assembler instruction From: Mary-Ann Johnson <MaryAnn.Johnson@di...> - 2007-11-07 16:08 >> r2 is not then used anywhere, so why does this happen, and is there >> anything I can do to stop it happening? > Try the latest snapshot version of SDCC. It should optimize this. The latest snapshot optimises it for &=3D writes, but it's still there = for |=3D writes: ; src/myfile.c:247: MY_REG |=3D 0xC0; mov dptr,#_MY_REG movx a,@dptr mov r3,a orl a,#0xC0 movx @dptr,a > The register was used during code generation as an > intermediate and then a peephole optimization rule removed > the read of it The .asm files generated by the latest snapshot don't seem to list the peephole optimisations used, so I can't tell where peephole optimisations are being used any more. Mary-Ann

Re: [Sdcc-user] Redundant assembler instruction From: Frieder Ferlemann <frieder.ferlemann@we...> - 2007-11-07 17:42 Hi Mary-Ann, Mary-Ann Johnson schrieb: > The .asm files generated by the latest snapshot don't seem to list the > peephole optimisations used, so I can't tell where peephole > optimisations are being used any more. use --fverbose-asm to get them again. Greetings, Frieder

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