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    <p>The reason is of course due to compilers optimizations, as already noted in the comments.</p> <p>Remembering that Objective-C is built on top of C, I put together a simple C example with different levels of optimizations and here's the result.</p> <h3>Original code</h3> <pre><code>int main(int argc, char const *argv[]) { char _isReadyForData = 0; while (!_isReadyForData); return 0; } </code></pre> <h3>LLVM IR with no optimizations (-O0)</h3> <pre><code>define i32 @main(i32 %argc, i8** %argv) #0 { entry: %retval = alloca i32, align 4 %argc.addr = alloca i32, align 4 %argv.addr = alloca i8**, align 8 %_isReadyForData = alloca i8, align 1 store i32 0, i32* %retval store i32 %argc, i32* %argc.addr, align 4 store i8** %argv, i8*** %argv.addr, align 8 store i8 0, i8* %_isReadyForData, align 1 br label %while.cond while.cond: ; preds = %while.body, %entry %0 = load i8* %_isReadyForData, align 1 %tobool = icmp ne i8 %0, 0 %lnot = xor i1 %tobool, true br i1 %lnot, label %while.body, label %while.end while.body: ; preds = %while.cond br label %while.cond while.end: ; preds = %while.cond ret i32 0 } </code></pre> <h3>LLVM IR with level 1 optimizations (-O1)</h3> <pre><code>define i32 @main(i32 %argc, i8** nocapture %argv) #0 { entry: br label %while.cond while.cond: ; preds = %while.cond, %entry br label %while.cond } </code></pre> <p>As you can see, the compiler produces an infinite loop when optimizing, since the local variable <code>_isReadyForData</code> is useless in that context and therefore is removed.</p> <p>As suggested by @faffaffaff, using the <code>volatile</code> keyword on <code>_isReadyForData</code> may solve the issue.</p> <h3>LLVM IR with level 1 optimizations (-O1) with volatile keyword</h3> <pre><code>define i32 @main(i32 %argc, i8** nocapture %argv) #0 { entry: %_isReadyForData = alloca i8, align 1 store volatile i8 0, i8* %_isReadyForData, align 1 br label %while.cond while.cond: ; preds = %while.cond, %entry %_isReadyForData.0.load1 = load volatile i8* %_isReadyForData, align 1 %lnot = icmp eq i8 %_isReadyForData.0.load1, 0 br i1 %lnot, label %while.cond, label %while.end while.end: ; preds = %while.cond ret i32 0 } </code></pre> <p>But I definitely agree with @rmaddy in saying that you'd better change the flow of your program and use driven logic, instead of patching what you have already.</p>
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    1. USGabriele Petronella
    1. USGabriele Petronella
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    1. COAwesome, thanks - question answered.
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