Igor Galić created TS-1278:
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Summary: Clang warns: Volatile fields read but results discarded
Key: TS-1278
URL: https://issues.apache.org/jira/browse/TS-1278
Project: Traffic Server
Issue Type: Bug
Components: Core
Environment: clang version 3.2 (trunk 157601)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Reporter: Igor Galić
{code}
Making all in eventsystem
make[2]: Entering directory
`/home/igalic/src/asf/trafficserver/BUILD/iocore/eventsystem'
CXX EventSystem.o
In file included from ../../../iocore/eventsystem/EventSystem.cc:31:
In file included from ../../../iocore/eventsystem/P_EventSystem.h:41:
In file included from ../../../iocore/eventsystem/I_EventSystem.h:35:
In file included from ../../../iocore/eventsystem/I_Action.h:30:
In file included from ../../../iocore/eventsystem/I_Continuation.h:40:
../../../iocore/eventsystem/I_Lock.h:404:19: error: expression result unused;
assign into a variable to force a volatile load
[-Werror,-Wunused-volatile-lvalue]
ink_assert(m->thread_holding);
~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
../../../lib/ts/ink_assert.h:54:31: note: expanded from macro 'ink_assert'
#define ink_assert(EX) (void)(EX)
^
1 error generated.
make[2]: *** [EventSystem.o] Error 1
{code}
Discussion from {{#llvm}} on IRC:
{code}
< jMCg> volatile EThreadPtr thread_holding;
<@baldrick> the clang warning sounds very sensible then
< jMCg>
http://git-wip-us.apache.org/repos/asf?p=trafficserver.git;a=blob;f=iocore/eventsystem/I_Lock.h#l80
< jMCg> The comment is great. "You must not modify or set this value directly."
-- that's why it's public!
< jMCg> baldrick: can you still help me understand it?
<@baldrick> jMCg: reading a volatile variable may have side-effects (that's
what volatile is for). For example, if it is mapped to some I/O area, each
read could fire off a nuclear warhead for example.
<@baldrick> jMCg: thus it seems sensible to warn if it looks like someone is
readying it but not using the result.
< jMCg> Oh. Now I'm back on track. *Reading* a volatile variable may have
*side-effects* - sometimes I'm slow.
< jMCg> baldrick: I'll open a Bug in our project. Thank you very much.
{code}
This is it.
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