Re: [Qemu-devel] [PATCH] target/mips: Support Toshiba specific three-operand MADD and MADDU

["Maciej W. Rozycki" <macro@linux-mips.org>]

On Sun, 14 Oct 2018, Philippe Mathieu-Daudé wrote:

> > > +            gen_move_low32(cpu_LO[acc], t2);
> > > +            gen_move_high32(cpu_HI[acc], t2);
> > > +            if (rd) {
> > > +                gen_move_low32(cpu_gpr[rd], t2);
> >
> > As above, are LO, HI and GPR[rd] sign-extended to 64 bits when required?
> 
>   MADDU rd, rs, rt
>   MADDU rs, rt
>   Multiply the contents of registers rs and rt as unsigned integers,
> and add the doubleword (64-bit)
>   result to multiply/divide registers HI and LO. Also, store the lower
> 32 bits of the add result in register
>   rd. In the MADDU rs, rt format, the store operation to a general
> register is omitted.
> 
> This one looks correct.

 Obviously with processors such as the TX49 or the TX79 where GPRs are 
64-bit for the purpose of integer arithmetic (i.e. any multimedia 
extension aside) any 32-bit results written to an integer register are 
implicitly sign-extended to the full 64-bit width of the destination 
register, as per the requirements of the MIPS architecture.  Otherwise 
operation of any subsequent 32-bit instruction (other than SLL or SLLV) 
using that that result as an input operand would be unpredictable.  You 
need to respect that in QEMU too.

 So results of individual operations are as in the comments with this 
code:

	mthi	$0		# HI <- 0
	mtlo	$0		# LO <- 0
	addiu	$2, $0, 1	# $2 <- 1
	lui	$3, 0x4000	# $3 <- 0x40000000
	maddu	$4, $3, $2	# HI <- 0
				# LO <- 0x40000000
				# $4 <- 0x40000000
	maddu	$5, $4, $2	# HI <- 0
				# LO <- 0xffffffff80000000
				# $5 <- 0xffffffff80000000
	maddu	$6, $4, $2	# HI <- 1
				# LO <- 0
				# $6 <- 0

Similarly:

	addiu	$2, $0, 1	# $2 <- 1
	sll	$3, $2, 31	# $3 <- 0xffffffff80000000
	dsll	$4, $2, 31	# $4 <- 0x80000000
	dsll	$5, $3, 0	# $5 <- 0xffffffff80000000
	sll	$6, $3, 0	# $6 <- 0xffffffff80000000
	dsll	$7, $4, 0	# $7 <- 0x80000000
	sll	$8, $4, 0	# $8 <- 0xffffffff80000000
	daddu	$9, $3, $3	# $9 <- 0xffffffff00000000
	addu	$10, $3, $3	# $10 <- 0
	daddu	$11, $4, $4	# $11 <- 0x100000000 
	addu	$12, $4, $4	# unpredictable!

 HTH,

  Maciej