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From: peterson@austin.ibm.com To: gcc-gnats@gcc.gnu.org Subject: c/10040: Incorrect warning message for bit fields Date: Wed, 12 Mar 2003 16:26:00 -0000 [thread overview] Message-ID: <20030312162142.18705.qmail@sources.redhat.com> (raw) >Number: 10040 >Category: c >Synopsis: Incorrect warning message for bit fields >Confidential: no >Severity: serious >Priority: medium >Responsible: unassigned >State: open >Class: rejects-legal >Submitter-Id: net >Arrival-Date: Wed Mar 12 16:26:00 UTC 2003 >Closed-Date: >Last-Modified: >Originator: James Peterson, IBM Austin Research Lab >Release: 3.2.2 >Organization: >Environment: RedHat Linux 7.1 on AMD Athlon >Description: warning: comparison is always 0 due to width of bit-field message is generated because a 1 bit field is tested against one: int _v: 1; if ((*tlb)._hi._v == 1) This message comes from around line 3132 in fold-const.c where a comment says "the constant shifted right by the field length should be all zero", and then proceeds to create trees for (1 >> (lbitsize-1)) and see if the result is zero (or all ones). But lbitsize is 1, so lbitsize-1 is zero and this quantity is then (1 >> 0) which is still 1, and hence non-zero (but is all one's to the size of the bit field, so maybe the problem is integer_all_onesp). This code is followed by explicit code dealing with "single-bit compares". In trying to explain the problem, I'm beginning to see that this error message is a result of the bit field being an "int" (signed) rather than "unsigned int". In which case, maybe the problem is best addressed by a different warning message -- "bit field too small for value and sign bit; consider unsigned type for field" >How-To-Repeat: >Fix: >Release-Note: >Audit-Trail: >Unformatted:
next reply other threads:[~2003-03-12 16:26 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2003-03-12 16:26 peterson [this message] 2003-03-12 16:45 nathan 2003-03-12 16:46 Falk Hueffner
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