Implement control flow statements in WebAssembly

[Sukolsak Sakshuwong]

This patch implements if, while, do, label, break, and continue statements in WebAssembly. Switches will be implemented in a subsequent patch.

[Sukolsak Sakshuwong]

Created attachment 260741 [details]
Patch

[Sukolsak Sakshuwong]

Created attachment 260742 [details]
Patch

[Sukolsak Sakshuwong]

When we implement an LLVM IR generator for WebAssembly, JumpTarget will be just an LLVM Basic Block. The code will look like this:

typedef LBasicBlock JumpTarget;

LBasicBlock newTarget()
{
    return m_out.newBlock();
}

void linkTarget(LBasicBlock target)
{
    m_out.jump(target);
    m_out.appendTo(target);
}
    
void jumpToTarget(LBasicBlock target)
{
    m_out.jump(target);
}
    
void jumpToTargetIf(JumpCondition condition, LValue expression, LBasicBlock target)
{
    LBasicBlock notTaken = m_out.newBlock();
    m_out.branch((condition == JumpCondition::Zero) ? m_out.isZero32(expression) : m_out.notZero32(expression), FTL::unsure(target), FTL::unsure(notTaken));
    m_out.appendTo(notTaken);
}

[Geoffrey Garen]

Comment on attachment 260742 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=260742&action=review

r=me

> Source/JavaScriptCore/wasm/WASMFunctionCompiler.h:407
> +    void endLabel()
> +    {
> +        linkTarget(m_breakLabelTargets.last());
> +
> +        m_breakLabelTargets.removeLast();
> +        m_continueLabelTargets.removeLast();
> +    }

Is it guaranteed that WASM can't have arbitrary gotos? (An arbitrary goto would make the push/pop model of labels invalid.)

[Sukolsak Sakshuwong]

Thanks!

(In reply to comment #4)
> Comment on attachment 260742 [details]
> Patch
> 
> View in context:
> https://bugs.webkit.org/attachment.cgi?id=260742&action=review
> 
> r=me
> 
> > Source/JavaScriptCore/wasm/WASMFunctionCompiler.h:407
> > +    void endLabel()
> > +    {
> > +        linkTarget(m_breakLabelTargets.last());
> > +
> > +        m_breakLabelTargets.removeLast();
> > +        m_continueLabelTargets.removeLast();
> > +    }
> 
> Is it guaranteed that WASM can't have arbitrary gotos? (An arbitrary goto
> would make the push/pop model of labels invalid.)

goto is not part of the MVP, but it is under consideration for a future version. <https://github.com/WebAssembly/design/blob/master/FutureFeatures.md#more-expressive-control-flow> The WASM format that we are using doesn't have goto.

[WebKit Commit Bot]

Comment on attachment 260742 [details]
Patch

Clearing flags on attachment: 260742

Committed r189514: <http://trac.webkit.org/changeset/189514>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.