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From: "jakub at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug inline-asm/20718] New: "+r" constraint with uninitialized value Date: Fri, 01 Apr 2005 16:39:00 -0000 [thread overview] Message-ID: <20050401163947.20718.jakub@gcc.gnu.org> (raw) unsigned long foo (unsigned long *a, unsigned long *b, unsigned long *c, int d) { unsigned long e, f; if (d <= 0) return 0; asm ("# registers %0 %1 %2 %3 %4 %5" : "+a" (e),"+c" (d), "+r" (f) : "r" (a), "r" (b), "r" (c)); return e; } at -O2 results in # registers %rax %ecx %rdx %rdi %rsi %rdx Note %2 and %5 being the same register, %rdx. If f (and e) are initialized, both older GCCs and GCC4+ assign different registers, but with unitialized values older GCCs seem to act as if "+r" was instead "=&r" while GCC4+ treat it as "=r". The library that used this (openssl) has been fixed to use "=&r" and "=&a" instead, but I'd like to understand what is the desirable GCC behaviour in this case. If it is undefined behaviour, so be it, but perhaps that should be documented. Similarly if "r" (unitialized_var) among inputs means GCC should allocate a unique register for that input or it doesn't have to. -- Summary: "+r" constraint with uninitialized value Product: gcc Version: 4.0.0 Status: UNCONFIRMED Severity: normal Priority: P2 Component: inline-asm AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: jakub at gcc dot gnu dot org CC: gcc-bugs at gcc dot gnu dot org,rth at gcc dot gnu dot org GCC target triplet: x86_64-*-linux http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20718
next reply other threads:[~2005-04-01 16:39 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2005-04-01 16:39 jakub at gcc dot gnu dot org [this message] 2005-04-01 19:19 ` [Bug inline-asm/20718] " pinskia at gcc dot gnu dot org 2005-07-21 21:57 ` pbrook at gcc dot gnu dot org
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