Messages in this thread | | | From | David Laight <> | Subject | RE: detecting integer constant expressions in macros | Date | Wed, 21 Mar 2018 10:35:58 +0000 |
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From: Uecker, Martin > Sent: 21 March 2018 10:22 > Am Mittwoch, den 21.03.2018, 10:51 +0100 schrieb Martin Uecker: > > > > Am Dienstag, den 20.03.2018, 17:30 -0700 schrieb Linus Torvalds: > > > On Tue, Mar 20, 2018 at 5:10 PM, Uecker, Martin > > > <Martin.Uecker@med.uni-goettingen.de> wrote: > > > > > > > But one could also use __builtin_types_compatible_p instead. > > > > > > That might be the right approach, even if I like how it only used > > > standard C (although _disgusting_ standard C) without it apart from > > > the small issue of sizeof(void) > > > > > > So something like > > > > > > #define __is_constant(a) \ > > > __builtin_types_compatible_p(int *, typeof(1 ? ((void*)((a) * 0l)) : (int*)1 ) ) > > > > > > if I counted the parentheses right.. > > > > This seems to work fine on all recent compilers. Sadly, it > > produces false positives on 4.4.7 and earlier when > > tested on godbolt.org > > > > Surprisingly, the MAX macro as defined below still seems > > to do the right thing with respect to avoiding the VLA > > even on the old compilers. > > > > I am probably missing something... or there are two > > compiler bugs cancelling out, or the __builting_choose_expr > > changes things. > > Nevermind, of course it avoids the VLA if it produces a false > positive and uses the simple version. So it is unsafe to use > on very old compilers.
False positives with old compilers don't matter when max() is being used for an on-stack array. The compilations with a new compiler will detect real VLA, the old compiler will generate valid code with a constant sized VLA.
OTOH these horrid: long buf[max(sizeof (struct foo), sizeof (struct bar)) + 7 / 8]; would be better replaced with: union buf { struct foo foo; struct bar bar; };
David
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