From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 17763 invoked by alias); 11 Jan 2013 14:52:41 -0000 Received: (qmail 17220 invoked by uid 48); 11 Jan 2013 14:52:05 -0000 From: "timo.kreuzer at reactos dot org" To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/37722] destructors not called on computed goto Date: Fri, 11 Jan 2013 14:52:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: middle-end X-Bugzilla-Keywords: wrong-code X-Bugzilla-Severity: normal X-Bugzilla-Who: timo.kreuzer at reactos dot org X-Bugzilla-Status: UNCONFIRMED X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Changed-Fields: CC Message-ID: In-Reply-To: References: X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated Content-Type: text/plain; charset="UTF-8" MIME-Version: 1.0 Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-bugs-owner@gcc.gnu.org X-SW-Source: 2013-01/txt/msg01031.txt.bz2 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37722 Timo Kreuzer changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |timo.kreuzer at reactos dot | |org --- Comment #3 from Timo Kreuzer 2013-01-11 14:52:03 UTC --- (In reply to comment #2) > int bar(int idx) { > static void* const gotos[] = {&&RETRY, &&INSIDE, &&OUTSIDE, &&EVIL}; > bool first = true; > { > RETRY: > foo<1> f1; > if(first) { > first = false; Well usually you cannot declare a variable after a label. That's a gcc extension. So you should either add curly braces between RETRY and INSIDE, or move RETRY above the preceeding brace. But that is just syntax, since it would be desired that it behaved like it would with a normal goto, even if that was placed like that. (I assume normal gotos will handle this properly) To theoretically solve the proplem, you could replace every indirect goto with code like this: { static void* const local_gotos[] = {&&label1, &&label2, &&label3, &&label4}; goto *local_gotos[idx]; label1: goto RETRY; label2: goto INSIDE; label3: goto OUTSIDE; label4: goto EVIL; } And now have the compiler optimize the pathes. This way, there are no more crazy jumps, just a very simple indirect jump and a number of normal jumps that the compiler should be able to handle anyway. The problem is now, that there might be multiple indirect jumps that use the same static data, which is just a bunch of void pointers. So depending on the origin of the jump different pathes would need to be generated for each target. This is incompatible with an indirect jump instruction though, which only utilizes some arbitrary address. One possible solution for this is to always invoke all destructors and constructors, even for local indirect gotos. So you would need to consider an indirect jump to always leave all scope blocks, up to the top level of the function and reenter from there. This would allow to generate labels/codepathes that are consistent for multiple indirect gotos. Another solution, is to use multiple jump tables. One table for each indirect goto containing one entry for each possibly referenced label in the function. The trick is that the original label addresses, that are produced with the && operator will all point to an "array" of direct jmp instructions. all of the same size. Now the first indirect goto would emit an indirect jump instruction to the address itself. The second one would substract the address of the first label, divide by the size of the jmp stub code and use the result as an index into it's own private jump table. Alternatively a static offset could be added to the actual address and multiple direct jump stubs would be generated in a row. table: .long label1, label2, label3, label4; table2: .long label1_from_2, label2_from_2, label3_from_2, label4_from_2; // first indirect goto mov eax, table[ecx * 4] // get the address from the table jmp eax // jump there // Second indirect goto mov ecx, table[ecx * 4] // get the address from the table sub ecx, label1 // substract the address of the first label shr ecx, 3 // divide by 8 (assuming each stub is 8 bytes) mov eax, table[ecx * 4] // get the address from the 2nd table jmp eax // jump there // Alternative: using multiple stub arrays mov eax, table[ecx * 4] // get the address from the table add eax, label5 - label1 // add the offset to the second "jmp array" jmp eax // jump there label1: jmp label1_from_1 label2: jmp label2_from_1 label3: jmp label3_from_1 label4: jmp label4_from_1 label5: jmp label1_from_2 label6: jmp label2_from_2 label7: jmp label3_from_2 label8: jmp label4_from_2 This mechanism would only be needed as soon as multiple indirect jumps could reference the same labels and different code pathes would need to be constructed for the targets depending on the origin of the goto. As an optimization it should be considered that labels, of which the address has been put in a table, which is now out of scope are not actually available anymore. Other optimizations might be using 2 different tables directly, if they are only used for 2 indirect jumps (someone might (mis)use it for different things like non-local gotos, exception handling, saving the address of code that is being executed for debugging purposes, etc)