[PATCH] D30810: Preserve vec3 type.

Akira Hatanaka via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Wed Mar 15 22:11:19 PDT 2017

ahatanak added a comment.

In https://reviews.llvm.org/D30810#701141, @jaykang10 wrote:

> In https://reviews.llvm.org/D30810#701132, @ahatanak wrote:
> > Actually, it's not a mis-compile. The record layout shows that there is a padding before field f2 and f2 starts at byte 16. So using "store <4 x float>" doesn't overwrite the field.
> It depends on float3's alignment. I guess the float3's alignment is 16 on your target so there is a padding bytes before f2 to be aligned by 16. If you add __attribute__((packed)) on struct type, you could see overwrite.

I looked at ItaniumRecordLayoutBuilder to understand how __attribute__((packed)) affects the struct's layout.

If the number of elements of a vector is not a power of 2, ASTContext::getTypeInfoImpl rounds up the width and alignment to the next power of 2. So the size of float3 is 16 bytes. __attribute__((packed)) can change the alignment of float3, but it doesn't change its size, so there is a 4-byte padding between field f1 and f2.


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