[gcc r17-3296] Limit the number of iterations in phi analyzer.

https://gcc.gnu.org/g:21c8ddd2cd2f3bf64225e4523e20e795083676fb

commit r17-3296-g21c8ddd2cd2f3bf64225e4523e20e795083676fb
Author: Andrew MacLeod <[email protected]>
Date:   Wed Aug 12 14:34:00 2026 -0400

    Limit the number of iterations in phi analyzer.
    
    The phi analyzer attempts to provide initial values for cyclic PHIs by
    simulating the modifer by the number of bits of precision.  _BitInt
    objects can be very large, and it is pointless to try to simulate them
    that many ttimes.
    
            PR tree-optimization/126538
            gcc/
            * gimple-range-phi.cc (phi_group::calculate_using_modifier): Limit the
            iteration upper bound to MAX_FIXED_MODE_SIZE.
    
            gcc/testsuite/
            * gcc.dg/pr126538.c: New.

Diff:
---
 gcc/gimple-range-phi.cc         |  2 +-
 gcc/testsuite/gcc.dg/pr126538.c | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/gcc/gimple-range-phi.cc b/gcc/gimple-range-phi.cc
index 82dd86a095a4..f680e87c7ee0 100644
--- a/gcc/gimple-range-phi.cc
+++ b/gcc/gimple-range-phi.cc
@@ -164,7 +164,7 @@ phi_group::calculate_using_modifier (range_query *q)
   // Limit iterations to 1 more than the number of bits.
   unsigned num_iter;
   if (do_iterative)
-    num_iter = TYPE_PRECISION (m_vr.type ()) + 1;
+    num_iter = MIN (TYPE_PRECISION (m_vr.type ()), MAX_FIXED_MODE_SIZE) + 1;
   else
     num_iter = 0;
 
diff --git a/gcc/testsuite/gcc.dg/pr126538.c b/gcc/testsuite/gcc.dg/pr126538.c
new file mode 100644
index 000000000000..7aae1d728e84
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr126538.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+typedef _BitInt(65535) T;
+
+__attribute__((noipa)) T
+f (int n)
+{
+  T x = 1;
+  for (int i = 0; i < n; i++)
+    x = x * 3;
+  return x;
+}
+
+int
+main (void)
+{
+  return (int) f (2) - 9;
+}