[LLVMdev] Inserting stack spilling code that modifies globally visible state

[Jakob Stoklund Olesen]

On Nov 9, 2011, at 4:20 AM, Jaroslav Sýkora wrote:

> Hello,
> I am developing compiler for the 8-bit PicoBlaze processor in LLVM 2.9.
> The problem at hand is that the stack spilling code in this
> architecture modifies the machine FLAGS register (i.e. globally
> visible state). The LLVM core code does not know about this 'feature'
> and happily inserts spilling code anywhere it wants, thus
> (potentially) corrupting live FLAGS.

Right, the register allocators expect to be able to insert spill/fill/copy instructions almost everywhere. (The only exception is after a terminator).

This will happen much more aggressively one you upgrade to 3.0.

> Practically, the FLAGS register is used only for multi-byte arithmetic
> (ADD+ADDCY pair), and conditional jumps (COMPARE+JUMP pair). Thus the
> FLAGS register live-range, which is correctly (I hope) expressed in
> the IR and it is visible to the reg. allocator and to all the existing
> machinery, is usually very short. All I need is that the spilling code
> would be never inserted into a live range of the FLAGS register. Is
> this possible?

Your best bet is to model these instruction sequences as VLIW bundles.

Currently, the best way of doing that is to create pseudo-instructions representing cmp+jmp etc.

> (As a side note: I tried to work-around the problem by moving the
> spilling code around "by hand" in
> TargetInstrInfoImpl::loadRegFromStackSlot / storeRegToStackSlot, but
> that is too late because physical registers are already allocated by
> then and thus it is not always possible to move the spilling code
> around without disturbing other register data dependencies.)

Exactly. That won't work.

/jakob