Alignment for Alloca Inst in llvm 2.6

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1

Hi,

  1. Does the alignment of the pointer returned by an AllocaInst depend on the instructions before it in the basic block? The 1st snippet below aligns ‘a’ correctly, while the 2nd one doesnt. (the method handle_args, only prints out the values of arguments, and returns argv unchanged).

define i32 @main(i32 %argc, i8** %argv) nounwind {
entry:
%argc_addr = alloca i32 ; <i32*> [#uses=1]
%argv_addr = alloca i8** ; <i8***> [#uses=2]
%retval = alloca i32 ; <i32*> [#uses=1]
%a = alloca [100 x i32], align 512 ; <[100 x i32]> [#uses=2]
%0 = call i8 @handle_args(i32 %argc, i8** %argv) ; <i8*> [#uses=1]
%argv_temp = bitcast i8* %0 to i8** ; <i8**> [#uses=1]

define i32 @main(i32 %argc, i8** %argv) nounwind {
entry:
%argc_addr = alloca i32 ; <i32*> [#uses=1]
%argv_addr = alloca i8** ; <i8***> [#uses=2]
%retval = alloca i32 ; <i32*> [#uses=1]
%0 = call i8* @handle_args(i32 %argc, i8** %argv) ; <i8*> [#uses=1]
%argv_temp = bitcast i8* %0 to i8** ; <i8**> [#uses=1]
%a = alloca [100 x i32], align 512 ; <[100 x i32]*> [#uses=2]

  1. I notice that the setAlignment method for an AllocaInst takes an unsigned. For 64 bit systems, alignment could potentially be larger. Is there a reason why the alignment should be restricted to this size?

Thanks in advance

Arushi

2

Would it ever be practical to align a stack variable to an 8 GiB
boundary? I'm curious what use case you could think of for aligning
to 64 KiB or more, since the unsigned is guaranteed to allow at least
0x8000.

3

I need objects aligned to the closest(largest) power of two boundary and the same case also applies to globals.

Arushi

4

Hello Arushi,

Are those listings your entire LLVM Assembly source? If so, you need to use a target datalayout if you have special alignment considerations on your platform. See http://llvm.org/docs/LangRef.html#datalayout for details.