llvm dev - Apr 2014 - [LLVMdev] Prevent sizeof being replaced by a constant in IR

Hello,

Using the following command:

clang -O0 -S -emit-llvm size.c

Will generate an LLVM IR file where uses of 'sizeof' have been replaced
by
constants. Is there any way to prevent this, and perhaps leave the
'sizeof's replaced by GEPs as detailed here:
http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt

Thanks,
Peter
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On 4/14/2014 8:35 AM, Peter Conn wrote:
> Hello,
>
> Using the following command:
>
> clang -O0 -S -emit-llvm size.c
>
> Will generate an LLVM IR file where uses of 'sizeof' have been
replaced
> by constants. Is there any way to prevent this, and perhaps leave the
> 'sizeof's replaced by GEPs as detailed here:
> http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt
"sizeof" _is_ a constant.

The technique from the note only works for aggregates that end with an 
array (or arrays themselves).

-Krzysztof

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation

No, there is no such clang option to control this.

Even if there were, the size would bleed through in lots of other places.
 For example, C++ features like templates and constexpr will require baking
in these sizes.

I also doubt LLVM IR supports array dimensions that aren't constant ints,
so the following code would have to use a constant:
struct foo { ... };
foo a1[NUMBER];
foo a2[sizeof(a1) / sizeof(foo)];


On Mon, Apr 14, 2014 at 6:35 AM, Peter Conn <conn.peter at gmail.com>
wrote:
> Hello,
>
> Using the following command:
>
> clang -O0 -S -emit-llvm size.c
>
> Will generate an LLVM IR file where uses of 'sizeof' have been
replaced by
> constants. Is there any way to prevent this, and perhaps leave the
> 'sizeof's replaced by GEPs as detailed here:
> http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt
>
> Thanks,
> Peter
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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