Hello List,
I've got a question about getelementptr.
I've been told that getelementptr is evaluated in compile-time NOT in run-time.
So I tested it with clang web compiler below.
http://llvm.org/demo/index.cgi
But with LTO nor Standard, neither one optimize getelementptr out.
Does getelementptr get evaluated in compile-time?
Or doesn't it get executed in run-time? (I believed so. Now I am wondering.)
Thank you in advance.
Hi Journeyer,
> I've got a question about getelementptr.
I've been told that getelementptr is evaluated in compile-time NOT in run-time.
this is impossible if the arguments are not constants. If they are constant,
then the optimizers will turn a getelementptr instruction into a getelementptr
constant expression (http://llvm.org/docs/LangRef.html#constant-expressions).
Even so it usually isn't evaluable at compile time, since typically the pointer
operand is a global variable for which the address is only known when the
program loader loads the final executable into memory. If however the operands
are all numerical values (eg the pointer operand is "null" and the others are
numbers like "1") then it will be turned into a number by the optimizers.
Ciao, Duncan.
Hello Duncan Sands,
If however the operands are all numerical values (eg the pointer operand is "null" and the others are numbers like "1") then it will be turned into a number by the optimizers.
I think my case is stated above among your answer.
http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt
There is a brief document about using getelmentptr trick above.
And I was wondering something similar to this case.
%Size = getelementptr %T* null, int 1
%SizeI = cast %T* %Size to uint
I understood that for this case, getelementptr will be collapsed to a
number during optimization. It is also explained in the brief document
above. I didn't understood by the document at that time.
OK. Thank you very much. I understood very well.
Sincerely
Journeyer
Hi Journeyer,
Hello Duncan Sands,
If however the operands are all numerical values (eg the pointer operand is "null" and the others are numbers like "1") then it will be turned into a number by the optimizers.
I think my case is stated above among your answer.
http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt
There is a brief document about using getelmentptr trick above.
And I was wondering something similar to this case.%Size = getelementptr %T* null, int 1
%SizeI = cast %T* %Size to uint
yes, this kind of thing will be turned into a number by the optimizers, assuming
that data layout information is available (since the optimizers need to know
the size of a %T to do this optimization).
Ciao, Duncan.