Bug 229459

Summary: useless warning using anonymous bitfield
Product: [Fedora] Fedora Reporter: Roland McGrath <roland>
Component: gccAssignee: Jakub Jelinek <jakub>
Status: CLOSED UPSTREAM QA Contact:
Severity: low Docs Contact:
Priority: medium    
Version: 5CC: aoliva
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Hardware: All   
OS: Linux   
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Fixed In Version: Doc Type: Bug Fix
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Last Closed: 2007-02-21 14:29:36 UTC Type: ---
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description Flags
test function to compile
none
Patch that fixes the bug by silencing the warning none

Description Roland McGrath 2007-02-21 10:31:39 UTC 
Description of problem:


Version-Release number of selected component (if applicable):
gcc-4.1.1-51.fc5

How reproducible:
100%

Steps to Reproduce:
1.gcc -O2 -Wall -S uninit-anon-bitfield.c
2.
3.
  
Actual results:

uninit-anon-bitfield.c: In function 'foo':
uninit-anon-bitfield.c:18: warning: 'tmp.<U48fc>' is used uninitialized in this
function

Expected results:

No warning, or at least a warning without a bogus field name.

Additional info:

In the case of an anonymous bitfield used for padding, there is no way the field
could be initialized.  There can be no harm from copying an uninitialized value
there, unless the target were part of a union that could access those bits in
another way.  At the very least, the warning is confusing because it names the
field with an internal identifier.  But really, there should be no warning when
there is no way to avoid it.
Comment 1 Roland McGrath 2007-02-21 10:31:40 UTC 
Created attachment 148471 [details]
test function to compile
Comment 2 Jakub Jelinek 2007-02-21 14:29:36 UTC 
A SRA bug.  Tracking this upstream, as it affects upstream 4.1/4.2/4.3 as well.
Comment 3 Alexandre Oliva 2007-03-05 07:35:11 UTC 
Created attachment 149247 [details]
Patch that fixes the bug by silencing the warning

It is simple enough to silence the warning.  Optimizing the code generated by
the compiler when the bitfields are scalarized is much harder, not only in
general (http://gcc.gnu.org/PR22156), but also for padding bitfields,
considering that padding bits *must* be copied unless the entire byte is a
padding byte (6.2.6.1/6 in C99-TC2).