Hi,
I am looking at some shootout benchmarks and I have the following
question. Look at the following program:
program loop;
{$mode objfpc}
var
d: array[0..$FFF] of double;
di: PDouble;
i: integer;
c: double;
begin
c := 125;
i := low(d);
repeat
d[i] := c*c;
inc(i);
until i>high(d);
di := @d[low(d)];
repeat
di^ := c*c;
inc(di);
until di>@d[high(d)]
end.
I am interested at the way the until clause is generated.
The compiler creates this assembler file:
# [15] d[i] := c*c;
fldl U_P$LOOP_C
fmul %st,%st
fstpl U_P$LOOP_D(,%edx,8)
fwait
# [16] inc(i);
incl %edx
# [17] until i>high(d);
cmpl $4095,%edx
jng .L9
# [18] di := @d[low(d)];
movl $U_P$LOOP_D,%ecx
.balign 4
.L16:
# [20] di^ := c*c;
fldl U_P$LOOP_C
fmul %st,%st
fstpl (%ecx)
fwait
# [21] inc(di);
addl $8,%ecx
# [22] until di>@d[high(d)]
movl $U_P$LOOP_D+32760,%eax
cmpl %ecx,%eax
jnb .L16
Line 17 generates two assembler instructions. Line 22 generates three
assembler instructions. As far as I can see both high(d) and @d[high(d)]
are constants, because d is a global variable.
Is it possible that line 22 will generate 2 assembler instructions in
the future? Or is this a limitation of the i386 assembler? Or do I make
a mistake in my reasoning?
Vincent
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