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Re: [patch] libstdc++/66017 Avoid bad casts and fix alignment of _Rb_tree_node<long long>::_M_storage

On 05/22/2015 09:13 AM, Jakub Jelinek wrote:
On Fri, May 22, 2015 at 03:59:47PM +0100, Jonathan Wakely wrote:
+      alignas(alignof(_Tp2)) unsigned char _M_storage[sizeof(_Tp)];

Is alignof(_Tp2) always the same as alignof(_Tp2::_M_t) on all targets
(I mean, won't some target align the structure more than its only field)?

Hmm, maybe. I don't know.

Wouldn't it be safer to use alignof(_Tp2::_M_t) here?


Though, apparently that is a GNU extension, so you'd need to use __alignof__

Yes, that's what I did in an earlier version of the patch, so I'll go
back to that.

Just grepped around, and e.g. on powerpc64le-linux -std=c++11 -malign-power -O2
typedef double _Tp;
struct _Tp2 { _Tp _M_t; };
extern _Tp2 tp2e;
int a = alignof(_Tp2);
int b = __alignof__(_Tp2::_M_t);
int c = alignof(_Tp);
int d = __alignof__(tp2e._M_t);
int e = alignof(_Tp2::_M_t);

we have a = 8, b = 4, c = 8, d = 4, e = 4.

Sorry to divert the discussion from the main topic but the output
above doesn't seem quite correct for this target.

The alignment of double is required to be 8 by the POWER ABIs so
the values of B, D, and E don't match the requirements or the
documentation of -malign-power(*). IBM xlC with the default
-qalign=power setting prints all 8 on POWER running AIX, as it
does with -qalign=natural, and I couldn't get it to match the
GCC result with any combination of alignment options.


[*] The manual says GCC follows the ABI rules by default, and
that the -malign-power option instructs GCC to follow the ABI-
specified alignment rules.

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