A minor issue with register allocator

[bryce at kampjes.demon.co.uk]

sig writes:
 > I'm currently testing my asm-parser. And found it generates unneeded
 > instruction (mov eax,eax):
 > code of asm method:
 > 
 > message4: arg1 with: arg2
 > 	| b c |
 > 	self pragma: #cdecl.
 > 	c := arg1 + arg2.
 > 	b := arg1 / arg2.
 > 	^ c + b
 > 
 > produced intermediate by parser:
 > #(#(#block 1
 > #(#mov #(#add #(#argument #arg1) #(#argument #arg2)) #c)
 > #(#mov #(#div #(#argument #arg1) #(#argument #arg2) 'temp7D6') #b)
 > #(#return #(#add #c #b))))
 > 
 > then with transformed arguments and cdecl prologue/epilogue:
 > #(#(#block 1 #(#push #ebp) #(#mov #esp #ebp)) #(#block 3
 >  #(#mov #(#add #(#mem #(#add #ebp -4)) #(#mem #(#add #ebp -8))) 't1')
 > #(#mov #(#div #(#mem #(#add #ebp -4)) #(#mem #(#add #ebp -8)) 't3') 't2')
 > #(#mov #(#add 't1' 't2') #eax) #(#jmp #block2)) #(#block 2 #(#mov #ebp
 > #esp) #(#pop #ebp) #(#ret)))
 > 
 > and then i using
 > LowLevelOptimiser ->InstructionSelector -> ColouringRegisterAllocator
 > -> JumpRemover :
 > 
 > #(#(#block 1 #(#push #ebp) #(#mov #esp #ebp)) #(#block 3 #(#mov #(-8
 > #ebp) #ebx) #(#add #(-4 #ebp) #ebx) #(#mov #(-4 #ebp) #eax) #(#mov
 > #(-8 #ebp) #ecx) #(#cdq) #(#div #ecx)
 > 
 > #(#mov #eax #eax)
 > 
 > #(#add #ebx #eax)) #(#block 2 #(#mov #ebp #esp) #(#pop #ebp) #(#ret)))
 > 
 > I suppose fix could be simple by testing in MedMov>>#visitWith: that
 > if reg1 = reg2 then return nil , which will be interpreted as no-op by
 > block.

I'm not sure why Exupery is doing that. The register uses a decent
move coalescer which will first try to create moves like (mov eax eax)
then remove them. The register allocator goes to some lengths to do
this.

It's possible that you've found a case where the no-op move removal
code isn't getting used, though I'm not sure where it is. It will be
interesting to here if the problem is still there after you change how
you're creating intermediate to make sure that there's only one #eax
register object. 1)

Exupery relies on object identity inside it's intermediate, this is
a design decision that's probabably in many places by now.

Bryce
 
1) Sig and I've spoken on irc about this.