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<p class="MsoNormal"><span style="font-family:"Courier New"">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">Using the builtin functions for atomic operations that honor the C++11 memory model requirements will result in undefined behavior if an illegal memory model is specified. Consequently, we can optimize-away
code that uses the result of one of these incorrectly invoked atomic ops.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">For example, for the function:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> type __atomic_load_n (type *ptr, int memmodel)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">the models __ATOMIC_RELEASE and __ATOMIC_ACQ_REL are not legal. So in a test-case like:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> int test_good(int* p_atomic, int* p1, int* p2) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> // __ATOMIC_RELAXED memmodel is legal for __atomic_load_n()<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> int v1 = *p1;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> int atomic_val = __atomic_load_n(p_atomic, __ATOMIC_RELAXED);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> int v2 = *p2;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> return v1 + v2 + atomic_val;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> int test_bad(int* p_atomic, int* p1, int* p2) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> // __ATOMIC_RELEASE memmodel is illegal for __atomic_load_n()<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> int v1 = *p1;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> int atomic_val = __atomic_load_n(p_atomic, __ATOMIC_RELEASE);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> int v2 = *p2;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> return v1 + v2 + atomic_val;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">since the 'test_bad()' function is using an illegal memmodel parameter, the behavior is undefined. Given that that function uses the result ('atomic_val') of that illegal atomic op in the return
expression, we notice that the return-value of the function is undefined, and therefore at -O2, we legitimately optimize-away anything that feeds into computing that return value.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">Prior to Clang 3.5, this illegal memmodel was accepted without a diagnostic (even with -Weverything). Beginning with 3.5, we produce:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> warning: memory order argument to atomic operation is invalid<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">So it's now better for users. But since we freely optimize-away computations that are based on the result of one of these illegal memmodel uses, it seems like it would be reasonable to treat it as
an error, rather than a warning.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">What do people think?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">FTR, trying this with g++ (version 4.8.2), shows that GNU considers it an error:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> error: invalid memory model for `__atomic_load'<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">-Warren<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">--<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">Warren Ristow<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">SN Systems - Sony Computer Entertainment Group<o:p></o:p></span></p>
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