On Mar 28, 2005, at 10:19, Raw Magick DOT COM wrote:
A := 0;
B := 0.9;
For X := 0 to 10000000 do
begin
A := A + X;
A := A * B;
end;
# Var A located at ebp-4
# Var B located at ebp-8
# Var X located at ebp-12
//A + B are set up before here - its the loop thats interrsting
# [44] For X := 0 to 10000000 do
movl $0,-12(%ebp)
decl -12(%ebp)
.balign 4
.L31:
incl -12(%ebp)
# [46] A := A + X;
flds -4(%ebp)
fildl -12(%ebp)
faddp %st,%st(1)
fstps -4(%ebp)
# [47] A := A * B;
flds -8(%ebp)
fmuls -4(%ebp)
fstps -4(%ebp)
cmpl $10000000,-12(%ebp)
jl .L31
The above code takes about 210ms to perform on my machine. Below is my
own assembler which takes about 100ms ( apologies it is in a slightly
different format )
asm
mov eax, 0; //Set up loop counter
@StartOfLoop:
mov dword ptr[x], eax; // Move its value into X ( on stack )
FILD dword ptr[x]; //Load into floating point
FADD dword ptr[A]; // Add A ( on Stack ) to it
FMUL dword ptr[B]; //Multiply by B ( on Stack )
FSTP dword ptr[A]; // Pop into A
add eax, 1; //Inc loop counter
cmp eax, 10000000; // Test Jump condition
jl @StartOfLoop;
end;
My question is, what needs to be done to the compiler to make it
optimise as well as C compilers, or perhaps I am missing some compiler
switches.
I expect you are using x86 family (may be even IA-32 architecture).
what about writing your code as:
A := 0;
B := 0.9;
For X := 10000000 downto 0 do
begin
A := A + X;
A := A * B;
end;
asm
mov ecx, 10000000; //Set up loop counter
@StartOfLoop:
mov dword ptr[x], ecx; // Move its value into X ( on stack )
FILD dword ptr[x]; //Load into floating point
FADD dword ptr[A]; // Add A ( on Stack ) to it
FMUL dword ptr[B]; //Multiply by B ( on Stack )
FSTP dword ptr[A]; // Pop into A
loop @StartOfLoop; //Dec loop counter
end;
May be
sub ecx, 1; //Dec loop counter
jnz @StartOfLoop;
instead of
loop @StartOfLoop;
can be better.
What are your results?
Variable alignment should have also a big influence.
for details see:
IA-32 Intel® Architecture Optimization Reference Manual
ftp://download.intel.com/design/Pentium4/manuals/24896611.pdf
Jan Ruzicka
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