|
![](/i/fill.gif) |
On 2001-02-06 13:17, Warp <war### [at] tag povray org> wrote:
> By the way, some times the compiler can go too far in this.
> This is the case with gcc. When generating the assembler code, it will
>always convert the multiplication of an integer variable and an integer
>constant to shifts and ors/additions/substractions, no matter what is the
>constant value.
> For example something like a*123456789 generates about 10-20 assembler
>operations.
Not always. E.g., egcs-2.91.66 for Intel will compile
int foo (int a) {
return a*123456789;
}
int bar (int a) {
return a*4;
}
into:
.file "foo.c"
.version "01.01"
gcc2_compiled.:
.text
.align 4
.globl foo
.type foo,@function
foo:
pushl %ebp
movl %esp,%ebp
imull $123456789,8(%ebp),%eax
leave
ret
.Lfe1:
.size foo,.Lfe1-foo
.align 4
.globl bar
.type bar,@function
bar:
pushl %ebp
movl %esp,%ebp
movl 8(%ebp),%edx
leal 0(,%edx,4),%eax
leave
ret
.Lfe2:
.size bar,.Lfe2-bar
.ident "GCC: (GNU) egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)"
As you can see, the first multiplication is translated to an imul
instruction, while the second one is translated to a leal (which is
essentially a shift-and-add operation.
As far as I can remember, gcc has behaved like this since at least the
1.3x releases.
hp
--
_ | Peter J. Holzer | All Linux applications run on Solaris,
|_|_) | Sysadmin WSR | which is our implementation of Linux.
| | | hjp### [at] wsr ac at |
__/ | http://www.hjp.at/ | -- Scott McNealy, Dec. 2000
Post a reply to this message
|
![](/i/fill.gif) |