>
> That's a great explanation Thomas.
> I'm curious though:Â how come both compilers produce this
> same sequence of instructions? I'd have thought it was a
> rather obscure combination. Is it perhaps more common
> than I'd suspected, or do GCC and Dignus have some common heritage
> in the back end?
No common heritage at all. The IBM compiler produces a similar
sequence.
Compiler writers are always looking for ways to do more using
less.
It's pretty well known, as are some other surprising sequences.
Here's one that will surprise people...
For this source:
int
foo(int x)
{
return x / 5;
}
the Dignus compiler (generating code for 32-bit z/OS)
generates:
* *** return x / 5;
L 15,0(0,1) ; x
LR 3,15 ; .
SRL 15,31(0) ; .
M 2,@lit_153_0 ; .
SRA 2,1(0) ; .
ALR 2,15 ; .
LR 15,2
...
DS 0D
@lit_153_0 DC F'1717986919' 0x66666667
which is correct, and avoids the division instruction.
It takes advantage of the fact the the M)ultiply instruction
uses 2 signed 32-bit operands and produces a signed 64-bit
result.
- Dave Rivers -
p.s. Even though we don't "know" the timing difference between
division and multiplication, it's a sure bet that division
takes a lot more time than multiplication on any hardware.
So, best to avoid it if you can.
--
riv...@dignus.com Work: (919) 676-0847
Get your mainframe programming tools at http://www.dignus.com
