<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Oct 22, 2010, at 10:59 AM, Marcin Świderski wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; font-size: medium; ">W dniu 22 października 2010 11:22 użytkownik Zhongxing Xu<span class="Apple-converted-space"> </span><span dir="ltr"><<a href="mailto:xuzhongxing@gmail.com">xuzhongxing@gmail.com</a>></span><span class="Apple-converted-space"> </span>napisał:<br><blockquote class="gmail_quote" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex; position: static; z-index: auto; ">When the binary operator is logical operator, why we need to replicate the control flow when adding the dtors in LHS and RHS? Could we just add the dtor right after where the LHS and RHS are evaluated?<br><br>That is, for code<br><br>A && B<br><br>we generate CFG like this:<br><br>A && B<br>~A()<br>| \<br>| B<br>| ~B()<br>| |<br>A && B<br><br>Since in logical binary operator, we only need the boolean value. The temporary object can be destroyed right after it is evaluated.<br><br></blockquote><div>This depends on how much we want to simulate real control flow of expression. C++ standard states that destructors of temporaries should be called at the end of full expression and in reverse order of their construction. Your example does not satisfy this. It only guaranties that destructor for temporary will be called.</div></span></blockquote></div><div><br></div><div>I think Zhongxing is right. Here is what the compiler does:</div><div><br></div><div><div>$ cat t.cpp</div><div><div>class A {</div><div>public:</div><div> A();</div><div> ~A();</div><div> operator bool();</div><div>};</div><div>class B {</div><div>public:</div><div> B();</div><div> ~B();</div><div> operator bool();</div><div>};</div><div><br></div><div>A foo();</div><div>B bar();</div><div><br></div><div>int test() {</div><div> return foo() || bar();</div><div>}</div></div></div><div><br></div><div>$ /Developer/usr/bin/clang -S t.cpp -O2 -fno-exceptions</div><div>$ cat t.s | c++filt</div><div>...</div><div><div><div>test(): ## @_Z4testv</div><div>Leh_func_begin0:</div><div>## BB#0: ## %entry</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>pushq<span class="Apple-tab-span" style="white-space:pre"> </span>%rbp</div><div>Ltmp0:</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>movq<span class="Apple-tab-span" style="white-space:pre"> </span>%rsp, %rbp</div><div>Ltmp1:</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>pushq<span class="Apple-tab-span" style="white-space:pre"> </span>%r14</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>pushq<span class="Apple-tab-span" style="white-space:pre"> </span>%rbx</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>subq<span class="Apple-tab-span" style="white-space:pre"> </span>$16, %rsp</div><div>Ltmp2:</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>leaq<span class="Apple-tab-span" style="white-space:pre"> </span>-24(%rbp), %rbx</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>movq<span class="Apple-tab-span" style="white-space:pre"> </span>%rbx, %rdi</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>callq<span class="Apple-tab-span" style="white-space:pre"> </span>foo()</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>movq<span class="Apple-tab-span" style="white-space:pre"> </span>%rbx, %rdi</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>callq<span class="Apple-tab-span" style="white-space:pre"> </span>A::operator bool()</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>testb<span class="Apple-tab-span" style="white-space:pre"> </span>%al, %al</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>movl<span class="Apple-tab-span" style="white-space:pre"> </span>$1, %ebx</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>jne<span class="Apple-tab-span" style="white-space:pre"> </span>LBB0_2</div><div>## BB#1: ## %temp.cond-dtor.call</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>leaq<span class="Apple-tab-span" style="white-space:pre"> </span>-32(%rbp), %rbx</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>movq<span class="Apple-tab-span" style="white-space:pre"> </span>%rbx, %rdi</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>callq<span class="Apple-tab-span" style="white-space:pre"> </span>bar()</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>movq<span class="Apple-tab-span" style="white-space:pre"> </span>%rbx, %rdi</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>callq<span class="Apple-tab-span" style="white-space:pre"> </span>B::operator bool()</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>movb<span class="Apple-tab-span" style="white-space:pre"> </span>%al, %r14b</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>movq<span class="Apple-tab-span" style="white-space:pre"> </span>%rbx, %rdi</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>callq<span class="Apple-tab-span" style="white-space:pre"> </span>B::~B()</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>movzbl<span class="Apple-tab-span" style="white-space:pre"> </span>%r14b, %ebx</div><div>LBB0_2: ## %temp.cond-dtor.cont</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>leaq<span class="Apple-tab-span" style="white-space:pre"> </span>-24(%rbp), %rdi</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>callq<span class="Apple-tab-span" style="white-space:pre"> </span>A::~A()</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>movl<span class="Apple-tab-span" style="white-space:pre"> </span>%ebx, %eax</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>addq<span class="Apple-tab-span" style="white-space:pre"> </span>$16, %rsp</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>popq<span class="Apple-tab-span" style="white-space:pre"> </span>%rbx</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>popq<span class="Apple-tab-span" style="white-space:pre"> </span>%r14</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>popq<span class="Apple-tab-span" style="white-space:pre"> </span>%rbp</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>ret</div><div>Leh_func_end0:</div></div></div><div><br></div><div><br></div><div>We can clearly see that there are 3 basic blocks and only 1 conditional branch. Notice that ~B() is called immediately after calling 'operator bool()' in class B.</div><div><br></div></body></html>