[cfe-dev] constant propagation produces undef

Alireza.Moshtaghi at microchip.com Alireza.Moshtaghi at microchip.com
Fri Jan 22 08:57:18 PST 2010

I'm sure I'm missing something in my port for PIC16 because the
following code produces undef for pic16 but it is ok for other ports but
I'm not sure what is it that I'm missing.

Looks like the constant propagation pass finds out that f() is always
returning 16 and it tries to substitute the call site in the shift with
statement 16. But since int is 16 bit it results in undef in the input
of check() function while zero is expected. 

If I modify f() to return 15 I will get the correct code. 

I was wondering if someone can give me a clue as what is it that I'm


By the way, this problem is happening on clang with optimizations on
(-O1). Without optimizations, things work fine.


extern int k; //int is 16 bit

extern int C;

int f(void)


  return 16;



void check(int );


int main (void)


  k = 1;

  C = k << f();

  check (C);

  return 0;



Produces :


@k = external global i16                          ; <i16*> [#uses=1]


define i16 @f() nounwind readnone {


  ret i16 16



define i16 @main() nounwind {


  store i16 1, i16* @k

  tail call void @check(i16 undef) nounwind

  ret i16 0






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