[swift-dev] SIL proposal: remove multiple results from SILValue

[John McCall]

> On Dec 4, 2015, at 9:06 AM, Erik Eckstein <eeckstein at apple.com> wrote:
> Hi,
> 
> currently some instructions, like alloc_stack and alloc_box (I think they are the only ones) return 2 values. For this we have the "result number" in SILValue, e.g. %3#0, %3#1
> 
> We could represent the same by using a projection instruction, e.g.
> 
> %3 = alloc_stack $Type  // == the old %3#0
> %4 = project_stack %1   // == the old %3#1
> 
> And actually we already have the project_box instruction.
> 
> This would simplify SILValues and many places in optimization passes/analysis. Yesterday I spent several hours to debug a problem where in some pass the wrong result number was used.
> 
> What do you think?

This would mean generating a lot of extra instructions for a lot of very common cases, like alloc_stack.

Was your bug related to the implicit conversion of a SILInstruction* -> SILValue that silently uses result 0?  I’ve been thinking for a while that we should make that conditional on the SILInstruction having exactly one result.

John.