[LLVMdev] optimizer returning wrong variable?

[Andrew Ferguson]

Sorry people, actually it looks like there's a problem with my logic! 
And even the raw .ll is running in the JIT (which is a first) 
Perhaps once I've got over the teething problems, I'll contribute some
documentation for using the c interface. 
Thanks for the help.  

On Mon, 2011-07-04 at 23:43 -0700, Nick Lewycky wrote:
> Andrew Ferguson wrote:
> >   I'm having some trouble trying to workout how to form functions from
> > the c interface
> > I thought I had it sorted but I guess I'm missing something or haven't
> > understood
> > the requirements, a case of trial and error and not really having a clue
> > to start with!
> >
> > I've got binary ops, cmps, for loops, while loops working but then it
> > hit the
> > wall with a tail cmp loop.
> >
> > looking at the output from the c interface it looks ok to me
> > but the optimizer is returning the wrong variable
> > I don't know what to do to avoid it?
> 
> I don't see the problem. In your input .ll, the function's return is:
> 
>    %c16 = load i32* %c
>    ret i32 %c16
> 
> The last store to %c is the one in Repeat2
> 
>    %add6 = add i32 %c, %add
>    store i32 %add6, i32* %c
> 
> which postdominates (ie., all routes from entry to exit must encounter 
> that store). Therefore we conclude that the return value is in fact %add6.
> 
> Are you actually seeing different behaviour for any particular set of 
> inputs on some target?
> 
> Nick
> 
> >
> > source language input
> >
> > Procedure TRU(a,b)
> > Protected x,y,c
> > Repeat
> > y=0
> > Repeat
> > c+x+y
> > y+1
> > Until y >= b
> > x+1
> > Until x >= a
> > ProcedureReturn c
> > EndProcedure
> >
> > output generated via the c Interface
> >
> > define i32 @TRU(i32 %a, i32 %b) {
> > Entry:
> > %a1 = alloca i32
> > store i32 %a, i32* %a1
> > %b2 = alloca i32
> > store i32 %b, i32* %b2
> > %c = alloca i32
> > store i32 0, i32* %c
> > %x = alloca i32
> > store i32 0, i32* %x
> > %y = alloca i32
> > store i32 0, i32* %y
> > br label %Repeat1
> >
> > Repeat1: ; preds = %Until1, %Entry
> > store i32 0, i32* %y
> > br label %Repeat2
> >
> > Repeat2: ; preds = %Until2, %Repeat1
> > %x3 = load i32* %x
> > %y4 = load i32* %y
> > %add = add i32 %x3, %y4
> > %c5 = load i32* %c
> > %add6 = add i32 %c5, %add
> > store i32 %add6, i32* %c
> > %y7 = load i32* %y
> > %add8 = add i32 %y7, 1
> > store i32 %add8, i32* %y
> > br label %Until2
> >
> > Until2: ; preds = %Repeat2
> > %y9 = load i32* %y
> > %b10 = load i32* %b2
> > %cond = icmp sge i32 %y9, %b10
> > br i1 %cond, label %Repeat2, label %Untilcmp2
> >
> > Untilcmp2: ; preds = %Until2
> > %x11 = load i32* %x
> > %add12 = add i32 %x11, 1
> > store i32 %add12, i32* %x
> > br label %Until1
> >
> > Until1: ; preds = %Untilcmp2
> > %x13 = load i32* %x
> > %a14 = load i32* %a1
> > %cond15 = icmp sge i32 %x13, %a14
> > br i1 %cond15, label %Repeat1, label %Untilcmp1
> >
> > Untilcmp1: ; preds = %Until1
> > %c16 = load i32* %c
> > ret i32 %c16
> > }
> >
> > verifying module
> > Optimize Function
> > Run in JIT
> > Jit Result: = 0
> > should = 325
> >
> > dump the optimized module
> >
> > define i32 @TRU(i32 %a, i32 %b) {
> > Entry:
> > br label %Repeat1
> >
> > Repeat1: ; preds = %Untilcmp2, %Entry
> > %c.0 = phi i32 [ 0, %Entry ], [ %add6, %Untilcmp2 ]
> > %x.0 = phi i32 [ 0, %Entry ], [ %add12, %Untilcmp2 ]
> > br label %Repeat2
> >
> > Repeat2: ; preds = %Repeat2, %Repeat1
> > %c.1 = phi i32 [ %c.0, %Repeat1 ], [ %add6, %Repeat2 ]
> > %y.0 = phi i32 [ 0, %Repeat1 ], [ %add8, %Repeat2 ]
> > %add = add i32 %c.1, %x.0
> > %add6 = add i32 %add, %y.0
> > %add8 = add i32 %y.0, 1
> > %cond = icmp slt i32 %add8, %b
> > br i1 %cond, label %Untilcmp2, label %Repeat2
> >
> > Untilcmp2: ; preds = %Repeat2
> > %add12 = add i32 %x.0, 1
> > %cond15 = icmp slt i32 %add12, %a
> > br i1 %cond15, label %Untilcmp1, label %Repeat1
> >
> > Untilcmp1: ; preds = %Untilcmp2
> > ret i32 %add6
> > }
> >
> > --
> > Andrew Ferguson
> >
> >
> >
> >
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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 

-- 
Andrew Ferguson
+649 372 6039
www.idlearts.com