About alloca instruction parameters

Beginners
#1

Hi, everyone, the llvm LangRef gives an explanation about alloca instruction.

syntax:

<result> = alloca [inalloca] <type> [, <ty> <NumElements>]

argument:

The ‘alloca’ instruction allocates sizeof(<type>)*NumElements bytes of memory on the runtime stack, returning a pointer of the appropriate type to the program.

My question is what “ty” means before NumElements parameter，since type has given the data type，is it the type of NumElements，could anybody help me, thanks.

#2

ty is the type of the NumElements SSA value.

(it is parsed here: https://github.com/llvm/llvm-project/blob/master/llvm/lib/AsmParser/LLParser.cpp#L7287 )

#3

@mehdi_amini, thank you, so it is the type of NumElements and NumElements is a SSA value, right?
in fact, what I have seen NumElements is just a number, constant.

%1 = alloca i32, i32 8

Could you tell me the way you find the code, maybe next time I can find answer in llvm src code by myself

#4

Constant is a subclass of Value in LLVM. You could have something else here, like here for example: https://github.com/llvm/llvm-project/blob/master/llvm/test/CodeGen/VE/Scalar/alloca.ll#L22

#5

OK, I get it, thank you. Could you tell me the way you find the code, maybe next time I can find answer in llvm src code by myself.

#6

git grep 'alloca.*%' llvm/test ; it is easier when you know what to look for :wink: