Re: Untagging by subtraction instead of masking on USE_LSB_TAG

[YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>]

>>>>> On Mon, 28 Jan 2008 13:25:30 +0900, Miles Bader <miles.bader@necel.com> said:

> Miles Bader <miles.bader@necel.com> writes:
>> Note that the important value here is the EA, not the offset.
>> Typically in a LSB tag scheme, the tags are arranged so that after
>> adding these "funny" offsets, the resulting EA is aligned properly.

> BTW, such a method won't work on architectures where offsets are
> inherently aligned -- for instance, doesn't the x86 implicitly scale
> the offset by the size of the value being loaded?  Does anybody know
> what is usually done on such an architecture?

I'm not sure, but there might be.  At least, the result on intel Mac
with gcc -Os was similar to that on PowerPC.

  _cons_to_long:                  _cons_to_long:
	  pushl   %ebp                    pushl   %ebp
	  movl    %esp, %ebp              movl    %esp, %ebp
	  movl    8(%ebp), %eax           movl    8(%ebp), %eax
	  testb   $7, %al                 testb   $7, %al
	  jne     L617                    jne     L617
	  sarl    $3, %eax                sarl    $3, %eax
	  jmp     L619                    jmp     L619
  L617:                           L617:
	  andl    $-8, %eax
	  movl    (%eax), %ecx            movl    -5(%eax), %ecx
	  movl    4(%eax), %edx           movl    -1(%eax), %edx
	  movl    %edx, %eax              movl    %edx, %eax
	  andl    $7, %eax                andl    $7, %eax
	  cmpl    $5, %eax                cmpl    $5, %eax
	  jne     L620                    jne     L620
	  movl    %edx, %eax
	  andl    $-8, %eax
	  movl    (%eax), %edx            movl    -5(%edx), %edx
  L620:                           L620:
	  movl    %ecx, %eax              movl    %ecx, %eax
	  sarl    $3, %eax                sarl    $3, %eax
	  sall    $16, %eax               sall    $16, %eax
	  sarl    $3, %edx                sarl    $3, %edx
	  orl     %edx, %eax              orl     %edx, %eax
  L619:                           L619:
	  popl    %ebp                    popl    %ebp
	  ret                             ret

And Solaris/SPARC with gcc -Os:

  cons_to_long:                   cons_to_long:
	  !#PROLOGUE# 0                   !#PROLOGUE# 0
	  !#PROLOGUE# 1                   !#PROLOGUE# 1
	  and     %o0, -8, %o4            mov     %o0, %o4
	  andcc   %o0, 7, %g0             andcc   %o0, 7, %g0
	  be      .LL952                  be      .LL949
	  sra     %o0, 3, %o0             sra     %o0, 3, %o0
	  ld      [%o4+4], %o5            ld      [%o4-1], %o5
	  and     %o5, 7, %g1             and     %o5, 7, %g1
	  cmp     %g1, 5                  cmp     %g1, 5
	  ld      [%o4], %g1              ld      [%o4-5], %g1
	  sra     %g1, 3, %g1             sra     %g1, 3, %g1
	  and     %o5, -8, %o3
	  bne     .LL954                  bne     .LL951
	  sll     %g1, 16, %o0            sll     %g1, 16, %o0
	  ld      [%o3], %o5              ld      [%o5-5], %o5
  .LL954:                         .LL951:
	  sra     %o5, 3, %g1             sra     %o5, 3, %g1
	  or      %o0, %g1, %o0           or      %o0, %g1, %o0
  .LL952:                         .LL949:
	  retl                            retl
	  nop                             nop

				     YAMAMOTO Mitsuharu
				mituharu@math.s.chiba-u.ac.jp