[fpc-pascal] fast integer multiplication
nickysn at SexMagnet.com
Thu Jul 28 21:46:15 CEST 2005
Florian Klaempfl wrote:
>Vincent Snijders wrote:
>>Suppose I have the following code:
>> a,b: dword;
>> c: qword;
>> a := 10000000;
>> b := 20000000;
>> c := a * b;
>>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:
>>#  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?
IMHO it should automatically optimize expressions like:
qword(a) * b
where a and b are dwords
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