Empty while loop hangs in iPhone release build

Question

For some reason having an empty while loop in a release build hangs, while having it in the debug build works fine. This example works in debug but hangs in release:

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Answer 1

The reason is of course due to compilers optimizations, as already noted in the comments.

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.

Original code

int main(int argc, char const *argv[])  {
    char _isReadyForData = 0;
    while (!_isReadyForData);
    return 0;
}

LLVM IR with no optimizations (-O0)

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
}

LLVM IR with level 1 optimizations (-O1)

define i32 @main(i32 %argc, i8** nocapture %argv) #0 {
entry:
  br label %while.cond

while.cond:                                       ; preds = %while.cond, %entry
  br label %while.cond
}

As you can see, the compiler produces an infinite loop when optimizing, since the local variable _isReadyForData is useless in that context and therefore is removed.

As suggested by @faffaffaff, using the volatile keyword on _isReadyForData may solve the issue.

LLVM IR with level 1 optimizations (-O1) with volatile keyword

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
}

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.