Fix ceil_log2(0) (PR 86644)

Fix ceil_log2(0) (PR 86644)

[Richard Sandiford]

This PR shows a pathological case in which we try SLP vectorisation on
dead code.  We record that 0 bits of the result are enough to satisfy
all users (which is true), and that led to precision being 0 in:

static unsigned int
vect_element_precision (unsigned int precision)
{
  precision = 1 << ceil_log2 (precision);
  return MAX (precision, BITS_PER_UNIT);
}

ceil_log2 (0) returned 64 rather than 0, leading to 1 << 64, which is UB.

Tested on aarch64-linux-gnu, aarch64_be-elf and x86_64-linux-gnu.
OK to install?

Richard


2018-07-24  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* hwint.c (ceil_log2): Fix comment.  Return 0 for 0.

Index: gcc/hwint.c
===================================================================
--- gcc/hwint.c	2018-05-02 08:38:14.433364094 +0100
+++ gcc/hwint.c	2018-07-24 19:09:03.522774662 +0100
@@ -60,12 +60,12 @@ floor_log2 (unsigned HOST_WIDE_INT x)
   return t;
 }
 
-/* Given X, an unsigned number, return the largest Y such that 2**Y >= X.  */
+/* Given X, an unsigned number, return the least Y such that 2**Y >= X.  */
 
 int
 ceil_log2 (unsigned HOST_WIDE_INT x)
 {
-  return floor_log2 (x - 1) + 1;
+  return x == 0 ? 0 : floor_log2 (x - 1) + 1;
 }
 
 /* Return the logarithm of X, base 2, considering X unsigned,

Re: Fix ceil_log2(0) (PR 86644)

[Jeff Law]

On 07/24/2018 12:11 PM, Richard Sandiford wrote:
> This PR shows a pathological case in which we try SLP vectorisation on
> dead code.  We record that 0 bits of the result are enough to satisfy
> all users (which is true), and that led to precision being 0 in:
> 
> static unsigned int
> vect_element_precision (unsigned int precision)
> {
>   precision = 1 << ceil_log2 (precision);
>   return MAX (precision, BITS_PER_UNIT);
> }
> 
> ceil_log2 (0) returned 64 rather than 0, leading to 1 << 64, which is UB.
> 
> Tested on aarch64-linux-gnu, aarch64_be-elf and x86_64-linux-gnu.
> OK to install?
> 
> Richard
> 
> 
> 2018-07-24  Richard Sandiford  <richard.sandiford@arm.com>
> 
> gcc/
> 	* hwint.c (ceil_log2): Fix comment.  Return 0 for 0.
OK
Jeff