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<p class="MsoNormal">Hi everyone,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Llvm detects zero pointer dereference in CFG simplify and insert intrinsic::trap (at least with –O1 or –O2).<o:p></o:p></p>
<p class="MsoNormal">Problem 1 : I find it unfortunate not to be able to disable it, and allow a specific target to handle it its own way (with MMU fault or dedicated HW stuff).<o:p></o:p></p>
<p class="MsoNormal">>> Is there a trick to avoid this ?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Problem 2 : Unless special care is taken in LLVM backend, DAG select will fail on trap, without a clear indication of the guilty C fragment<o:p></o:p></p>
<p class="MsoNormal">>> Should the frontend emit at least a warning ?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks for your help.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">--<o:p></o:p></p>
<p class="MsoNormal">Fred<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Example :<o:p></o:p></p>
<p class="MsoNormal">C code (many lines remove for the sake of clarity) :<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">struct s;<o:p></o:p></p>
<p class="MsoPlainText">struct s{<o:p></o:p></p>
<p class="MsoPlainText"> struct s *next;<o:p></o:p></p>
<p class="MsoPlainText">};<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">struct s* foo (struct s *a){<o:p></o:p></p>
<p class="MsoPlainText"> struct s *b = 0;<o:p></o:p></p>
<p class="MsoPlainText"> b->next = a; <o:p></o:p></p>
<p class="MsoPlainText"> return b;<o:p></o:p></p>
<p class="MsoPlainText">}<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">LLVM (clang –O2 –emit-llvm) :<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">%struct.s = type { %struct.s* }<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">; Function Attrs: noreturn nounwind<o:p></o:p></p>
<p class="MsoPlainText">define noalias %struct.s* @foo(%struct.s* nocapture readnone %a) #0 {<o:p></o:p></p>
<p class="MsoPlainText">entry:<o:p></o:p></p>
<p class="MsoPlainText"> tail call void @llvm.trap()<o:p></o:p></p>
<p class="MsoPlainText"> unreachable<o:p></o:p></p>
<p class="MsoPlainText">}<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">; Function Attrs: noreturn nounwind<o:p></o:p></p>
<p class="MsoPlainText">declare void @llvm.trap() #1<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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