On Thu, Dec 02, 2021 at 08:53:31AM -0500, Vladimir Makarov via Gcc-patches
wrote:
> The following patch fixes
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103437
>
> The patch was successfully bootstrapped and tested on x86-64. There is no
> test as the bug occurs on GCC built with sanitizing for an existing go test.
> commit c6cf5ac1522c54b2ced98fc687e973a9ff17ba1e
> Author: Vladimir N. Makarov <vmaka...@redhat.com>
> Date: Thu Dec 2 08:29:45 2021 -0500
>
> [PR103437] Process multiplication overflow in priority calculation for
> allocno assignments
>
> We process overflows in cost calculations but for huge functions
> priority calculation can overflow as priority can be bigger the cost
> used for it. The patch fixes the problem.
>
> gcc/ChangeLog:
>
> PR rtl-optimization/103437
> * ira-color.c (setup_allocno_priorities): Process multiplication
> overflow.
>
> diff --git a/gcc/ira-color.c b/gcc/ira-color.c
> index 3d01c60800c..1f80cbea0e2 100644
> --- a/gcc/ira-color.c
> +++ b/gcc/ira-color.c
> @@ -2796,7 +2796,7 @@ static int *allocno_priorities;
> static void
> setup_allocno_priorities (ira_allocno_t *consideration_allocnos, int n)
> {
> - int i, length, nrefs, priority, max_priority, mult;
> + int i, length, nrefs, priority, max_priority, mult, diff;
> ira_allocno_t a;
>
> max_priority = 0;
> @@ -2807,11 +2807,14 @@ setup_allocno_priorities (ira_allocno_t
> *consideration_allocnos, int n)
> ira_assert (nrefs >= 0);
> mult = floor_log2 (ALLOCNO_NREFS (a)) + 1;
> ira_assert (mult >= 0);
> - allocno_priorities[ALLOCNO_NUM (a)]
> - = priority
> - = (mult
> - * (ALLOCNO_MEMORY_COST (a) - ALLOCNO_CLASS_COST (a))
> - * ira_reg_class_max_nregs[ALLOCNO_CLASS (a)][ALLOCNO_MODE (a)]);
> + mult *= ira_reg_class_max_nregs[ALLOCNO_CLASS (a)][ALLOCNO_MODE (a)];
> + diff = ALLOCNO_MEMORY_COST (a) - ALLOCNO_CLASS_COST (a);
> + /* Multiplication can overflow for very large functions.
> + Check the overflow and constrain the result if necessary: */
> + if (__builtin_smul_overflow (mult, diff, &priority)
I'm afraid we can't use __builtin_smul_overflow, not all system compilers
will have that.
But, as it is done in int and we kind of rely on int being 32-bit on host
and rely on long long being 64-bit, I think you can do something like:
long long priorityll = (long long) mult * diff;
priority = priorityll;
if (priorityll != priority
...
> + || priority <= -INT_MAX)
> + priority = diff >= 0 ? INT_MAX : -INT_MAX;
> + allocno_priorities[ALLOCNO_NUM (a)] = priority;
> if (priority < 0)
> priority = -priority;
> if (max_priority < priority)
Jakub
