Is a series of fields in a structure guaranteed to have the same layout
as those fields in a structure on their own? That is, can I cast a
pointer within a main structure to an equivalent type?
X = { a, b, c, d, e }
Y = { c, d, e }
y = BitCast( StructGEP( some_x, 2 ), Y* )
Is that a valid cast?
--
edA-qa mort-ora-y
Leaf Creator
Leaf - the language we always wanted
http://leaflang.org/
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] > On Behalf Of edA-qa mort-ora-y > Subject: [LLVMdev] struct alignment question> Is a series of fields in a structure guaranteed to have the same layout > as those fields in a structure on their own? That is, can I cast a > pointer within a main structure to an equivalent type?> X = { a, b, c, d, e } > Y = { c, d, e }> y = BitCast( StructGEP( some_x, 2 ), Y* )> Is that a valid cast?In general, no. If a, b, and c were char, and d was an int, using typical C alignments, there would be one slack byte between c and d in X, whereas there would be three in Y. You could probably force what you want with packed structures or by playing with the data layout setting. - Chuck
On 19/11/13 08:03, Caldarale, Charles R wrote:>> X = { a, b, c, d, e } >> Y = { c, d, e }> In general, no. If a, b, and c were char, and d was an int, using > typical C alignments, there would be one slack byte between c and d > in X, whereas there would be three in Y. You could probably force > what you want with packed structures or by playing with the data > layout setting.Yes, I forgot about the simple scenario. If I force the alignment on "c" should I be okay? That is, if I force it to the maximum alignment for the platform will { c, d, e } always have the same layout? -- edA-qa mort-ora-y Leaf Creator Leaf - the language we always wanted http://leaflang.org/