llvm dev - Oct 2021 - verifyFunction fails validation with Terminator found in the middle of a basic block

Hi,

I'm generating IR for a factorial function that looks like below. When I
call the verifyFunction, it fails validation with the error
message Terminator found in the middle of a basic block. label %then

The resulting IR still works fine but I'm not sure if the code generation
should be done in a better way for the if/then block below.

Thanks in advance,
Vijay

define double @fact(double %n) {
entry:
  %n1 = alloca double, align 8
  store double %n, double* %n1, align 8
  %n2 = load double, double* %n1, align 8
  %cmptmp = fcmp ule double %n2, 1.000000e+00
  %booltmp = uitofp i1 %cmptmp to double
  %ifcond = fcmp one double %booltmp, 0.000000e+00
  br i1 %ifcond, label %then, label %ifcont

then:                                             ; preds = %entry
  ret double 1.000000e+00
  br label %ifcont

ifcont:                                           ; preds = %then, %entry
  %n3 = load double, double* %n1, align 8
  %n4 = load double, double* %n1, align 8
  %subtmp = fsub double %n4, 1.000000e+00
  %calltmp = call double @fact(double %subtmp)
  %multmp = fmul double %n3, %calltmp
  ret double %multmp
}
Validating function...
Terminator found in the middle of a basic block!
label %then
Validation failed
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On Wed, Oct 20, 2021 at 12:52 PM Vijay via llvm-dev <llvm-dev at
lists.llvm.org>
wrote:
> Hi,
>
> I'm generating IR for a factorial function that looks like below. When
I
> call the verifyFunction, it fails validation with the error
> message Terminator found in the middle of a basic block. label %then
>
Because there shouldn't be a terminator instruction (in this case the
return instruction) in the middle of a basic block. I see no problem with
the verifier's decision here.
I think the %then block should only contain the return instruction. The `br
label %ifcont` instruction is redundant because the control flow will not
proceed anyway beyond the return instruction once %then block is taken.

-Min

>
> The resulting IR still works fine but I'm not sure if the code
generation
> should be done in a better way for the if/then block below.
>
> Thanks in advance,
> Vijay
>
> define double @fact(double %n) {
> entry:
>   %n1 = alloca double, align 8
>   store double %n, double* %n1, align 8
>   %n2 = load double, double* %n1, align 8
>   %cmptmp = fcmp ule double %n2, 1.000000e+00
>   %booltmp = uitofp i1 %cmptmp to double
>   %ifcond = fcmp one double %booltmp, 0.000000e+00
>   br i1 %ifcond, label %then, label %ifcont
>
> then:                                             ; preds = %entry
>   ret double 1.000000e+00
>   br label %ifcont
>
> ifcont:                                           ; preds = %then, %entry
>   %n3 = load double, double* %n1, align 8
>   %n4 = load double, double* %n1, align 8
>   %subtmp = fsub double %n4, 1.000000e+00
>   %calltmp = call double @fact(double %subtmp)
>   %multmp = fmul double %n3, %calltmp
>   ret double %multmp
> }
> Validating function...
> Terminator found in the middle of a basic block!
> label %then
> Validation failed
>
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> llvm-dev at lists.llvm.org
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>

-- 
Min-Yih Hsu
Ph.D Student in ICS Department, University of California, Irvine (UCI).
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