Based on Richi's patch:
<https://gcc.gnu.org/pipermail/gcc-patches/2024-April/648725.html>
~~
The following avoids re-walking and re-combining the instructions
between i2 and i3 when the pattern of i2 doesn't change.

Bootstrap and regtest running ontop of a reversal of
r14-9692-g839bc42772ba7a.

It brings down memory use frmo 9GB to 400MB and compile-time from
80s to 3.5s.  r14-9692-g839bc42772ba7a does better in both metrics
but has shown code generation regressions across acrchitectures.

	PR rtl-optimization/101523
	* combine.cc (try_combine): When the pattern of i2 doesn't
	change do not re-start combining at i2 or an earlier insn which
	had links or notes added.
~~
But, since the patch affects code generation (for instance,
libstdc++-v3/src/c++17/floating_from_chars.o), we limit the bailing out
only when I2 hasn't been changed 1000x.  I've measured how many times
at most is I2 unchanged during a bootstrap + regtest.
x86: 134
aarch64: 736 (gimple-match-1.cc)
s390x: 635 (gimple-match-*)
ppc64le: 620 (gimple-match-*)
while certain pathological testcases trigger it more than 10,000 times.
With the limit in place this patch doesn't affect common code.

--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -92,6 +92,11 @@ along with GCC; see the file COPYING3.  If not see
 #include "function-abi.h"
 #include "rtlanal.h"
 
+/* Number of times I2 didn't change in try_combine.  Used to prevent a
+   combinatorial explosion.  */
+
+static int combine_unchanged;
+
 /* Number of attempts to combine instructions in this function.  */
 
 static int combine_attempts;
@@ -1127,6 +1132,7 @@ combine_instructions (rtx_insn *f, unsigned int nregs)
     return false;
 
   combine_attempts = 0;
+  combine_unchanged = 0;
   combine_merges = 0;
   combine_extras = 0;
   combine_successes = 0;
@@ -4196,6 +4201,10 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
       adjust_for_new_dest (i3);
     }
 
+  bool i2_unchanged = false;
+  if (rtx_equal_p (newi2pat, PATTERN (i2)))
+    i2_unchanged = true;
+
   /* We now know that we can do this combination.  Merge the insns and
      update the status of registers and LOG_LINKS.  */
 
@@ -4762,6 +4771,13 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
   combine_successes++;
   undo_commit ();
 
+  if (i2_unchanged)
+    {
+      if (combine_unchanged == 1000)
+	return i3;
+      ++combine_unchanged;
+    }
+
   rtx_insn *ret = newi2pat ? i2 : i3;
   if (added_links_insn && DF_INSN_LUID (added_links_insn) < DF_INSN_LUID (ret))
     ret = added_links_insn;