[LLVMdev] optimizer returning wrong variable?

[Nick Lewycky]

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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