Florian Klaempfl a écrit :
Vincent Snijders wrote:
Hi,
Suppose I have the following code:
var
a,b: dword;
c: qword;
begin
a := 10000000;
b := 20000000;
c := a * b;
writeln(c);
end.
Now, although c is large enough to contain the result only the lower
dword is filled. I can force correct results by using c := qword(a) * b,
but the slow fpc_mul_qword is used.
The code generated for the above sample is:
# [16] c:=a*b;
movl U_P$PROJECT1_A,%edx
movl U_P$PROJECT1_B,%eax
mull %edx
movl $0,%edx
movl %eax,U_P$PROJECT1_C
movl %edx,U_P$PROJECT1_C+4
What I want is the above code, but without the "movl $0,%edx"
instruction. Is there a way to do this (wihtout using fpc_mul_qword).
Only assembler for now. Any suggestions how the compiler could be told
to generate such code?
Since you ask the question, I suppose you cannot simply
suppress the "movl $0,%edx" line generation [*]. What about
using a compiler directive, something like {$EXTENDEDMUL ON/OFF}
for instance?
[*] Note that because of this line, currently FPC has not a
"coherent" behaviour. If you write x := y*z with x: Word and
y,z: Byte or with x: Longword and y,z: Word, the resulting value
x is the exact multiplication result and not the result truncated
to the size of the multiplicand and multiplicator type (like it
is with x: QWord and y,z: Longword).
MM
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