llvm dev - Mar 2011 - [LLVMdev] compiler-rt: Infinite loop/stack overflow in __modsi3()

Hi All,
     The default implementation of __modsi3() (signed integer modulus) 
in compiler-rt/lib/modsi3.c is defined recursively.  Thankfully, LLVM is 
smart enough to do tail call elimination on the recursion, so I got an 
infinite loop rather than a stack overflow :)

     Here's the patch, patterned after the correct implementation in 
umodsi3.c:

diff --git a/lib/compiler-rt/lib/modsi3.c b/lib/compiler-rt/lib/modsi3.c
index 388418a..3759ce0 100644
--- a/lib/compiler-rt/lib/modsi3.c
+++ b/lib/compiler-rt/lib/modsi3.c
@@ -16,8 +16,10 @@

  /* Returns: a % b */

+su_int __divsi3(si_int a, si_int b);
+
  si_int
  __modsi3(si_int a, si_int b)
  {
-    return a - (a / b) * b;
+    return a - __divsi3(a, b) * b;
  }


Best,
Matt

On Mar 10, 2011, at 11:31 AM, Matt Johnson wrote:
> Hi All,
>     The default implementation of __modsi3() (signed integer modulus) 
> in compiler-rt/lib/modsi3.c is defined recursively.  Thankfully, LLVM is 
> smart enough to do tail call elimination on the recursion, so I got an 
> infinite loop rather than a stack overflow :)
Looks good, applied in r127429, thanks!

-Chris
> 
>     Here's the patch, patterned after the correct implementation in 
> umodsi3.c:
> 
> diff --git a/lib/compiler-rt/lib/modsi3.c b/lib/compiler-rt/lib/modsi3.c
> index 388418a..3759ce0 100644
> --- a/lib/compiler-rt/lib/modsi3.c
> +++ b/lib/compiler-rt/lib/modsi3.c
> @@ -16,8 +16,10 @@
> 
>  /* Returns: a % b */
> 
> +su_int __divsi3(si_int a, si_int b);
> +
>  si_int
>  __modsi3(si_int a, si_int b)
>  {
> -    return a - (a / b) * b;
> +    return a - __divsi3(a, b) * b;
>  }
> 
> 
> Best,
> Matt
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