FTL: a + b overflow check shouldn't keep both a and b alive on the exit path if they wouldn't have otherwise both been live

• Component: JavaScriptCore
• Status: RESOLVED DUPLICATE

[Filip Pizlo]

Consider that we have a program like:

    return a + b

Currently we'll emit code like:

    movl %ecx, %rax
    addl %edx, %rax
    jo slow

Note the movl.  That's there because the OSR exit slow path will want both a and b, and so the result of the addition must go into a different register than either of the inputs.

But we could fix that if the OSR exit path undid the addition:

    %result = @llvm.sadd.with.overflow(%a, %b)
    if (extract %result, 1)
        exit(%a, %result - %a)

In this case, we can do an addl that destroys %b.

I've written some code at the FTL lowering level that does this, and it does seem to remove a bunch of movl's from the code for V8v7/crypto's am3() function.  But it's not an overall speed-up.  I suspect it would be better to do this optimization in LLVM.  It's pretty weird to do this at the time of LLVM IR generation.

[Filip Pizlo]

Created attachment 219271 [details]
the patch

I don't think this works yet.

[Filip Pizlo]

*** This bug has been marked as a duplicate of bug 126545 ***