On Thu, Jul 24, 2025 at 8:40 AM Richard Biener
<richard.guent...@gmail.com> wrote:
>
> On Thu, Jul 24, 2025 at 2:39 AM magic0826gc via Gcc <gcc@gcc.gnu.org> wrote:
> >
> > Dear GCC Developers,
> > I'm writing to report an observation regarding loop unswitching behavior
> > when compiling the attached code with GCC 15 using -O2 -funswitch-loops
> > optimization flags. Here's a detailed analysis of the issue:
> > Code Overview:
> > int
> > foo(double *a, double *b, double *c, double *d, double *r, int size, int
> > order)
> > {
> > for (int i = 0; i < size; i++)
> > {
> > double tmp;
> > if (order < 3)
> > tmp = -8 * a[i];
> > else
> > tmp = -4 * b[i];
> > double x = 3 * tmp + d[i] + tmp;
> > if (5 > order)
> > x = 2;
> > if (order == 12345)
> > x = 5;
> > for (int j = 0; j < size; j++)
> > {
> > double y = 3.4f * tmp + d[i];
> > r[j] += x + y;
> > }
> > }
> > return 0;
> > }
> > Outer loop with multiple conditional checks on 'order' parameter.
> > Complex control flow with three independent conditionals.
> > Inner loop containing runtime-invariant computations.
> > Observed Behavior:
> > The compiler fails to unswitch the outer loop despite the presence of
> > multiple loop-invariant conditions. This prevents:
> > Constant propagation of 'x' variable. Elimination of dead code paths (order
> > == 12345 and 5 > order check).
> > Current unswitching (tree_unswitch_outer_loop) appears limited to inner
> > loop guards. What prevents handling nested loops with complex control flow?
> > Potential Constraints:
> > Register pressure considerations in multi-versioned loops. Code size
> > explosion vs. runtime tradeoffs. Interaction with subsequent optimization
> > passes.
> > Could you help clarify:
> > Architectural constraints in current unswitching implementation?
> > Planned improvements for nested loop optimization?
>
> Unswitching does currently not implement unswitching of non-innermost
> loops. A quick
> search in bugzilla doesn't reveal a bug where the summary notes this
> so I suggest
> you file a bugzilla for this.
Actually I stand corrected - I implemented unswitching of outer loops
a few years ago,
but it only unswitches inner loop invariant conditions, never on
conditions that appear
in an outer loop.
> > Recommended workarounds for such patterns?
>
> You'd have to manually unswitch the outer loop.
>
> Richard.
>
> > Thank you for your time and expertise. I'd be happy to provide additional
> > details if needed.
> > Best regards,
