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From: "yann at droneaud dot fr" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/57908] alignment of arrays allocated stack on amd64/x86_64: 16 bytes ?
Date: Tue, 16 Jul 2013 15:32:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57908

--- Comment #8 from Yann Droneaud <yann at droneaud dot fr> ---
Using -Os show different results:

   Arrays                                                                
   object |       u8_0 |     0x7fff9b4c05bc |    1 |         4 |        1
   object |       u8_1 |     0x7fff9b4c05c7 |    3 |         1 |        1
   object |       u8_2 |     0x7fff9b4c05da |    7 |         2 |        1
   object |       u8_3 |     0x7fff9b4c05d0 |    5 |        16 |        1
   object |       u8_4 |     0x7fff9b4c05bf |    2 |         1 |        1
   object |       u8_5 |     0x7fff9b4c05bd |    1 |         1 |        1
   object |       u8_6 |     0x7fff9b4c05ca |    3 |         2 |        1
   object |       u8_7 |     0x7fff9b4c05d5 |    5 |         1 |        1
   object |       u8_8 |     0x7fff9b4c05e1 |    7 |         1 |        1
   object |       u8_9 |     0x7fff9b4c05c1 |    2 |         1 |        1
   object |      u8_10 |     0x7fff9b4c05e8 |   11 |         8 |        1
   object |      u8_11 |     0x7fff9b4c05cd |    3 |         1 |        1
   object |      u8_12 |     0x7fff9b4c05f3 |   13 |         1 |        1
   object |      u8_13 |     0x7fff9b4c05c3 |    2 |         1 |        1
   object |      u8_14 |     0x7fff9b4c05be |    1 |         2 |        1
   object |      u8_15 |     0x7fff9b4c05c5 |    2 |         1 |        1


So GCC is using 16 bytes to align array allocated on stack by default but it's
not enforcing such alignment.
I guess it's all about optimisation ... but wasting up to 14 bytes to get an
array of 2 bytes aligned might be overkill.

Could someone comment on which optimisation is achieved by aligning such small
arrays ?

Regards.