[gcc r14-12748] match.pd: Guard x+x -> x*2 simplification [PR126257]

https://gcc.gnu.org/g:097433757515c05313a6e26d1b87cf1b5758aebc

commit r14-12748-g097433757515c05313a6e26d1b87cf1b5758aebc
Author: Jakub Jelinek <[email protected]>
Date:   Wed Jul 15 10:27:56 2026 +0200

    match.pd: Guard x+x -> x*2 simplification [PR126257]
    
    The x+x -> x*2 simplication obviously requires that 2 is representable
    in type, so that rules out unsigned _BitInt(1) and signed _BitInt(2)
    (and 1 too in C2Y, ditto unsigned:1 and signed:2 and :1), otherwise
    we don't multiply by 2 but by 0 or -2.
    While perhaps we could transform in those cases x+x to x<<1, I'm not
    convinced it is worth it.
    
    2026-07-15  Jakub Jelinek  <[email protected]>
    
            PR tree-optimization/126257
            * match.pd (x+x -> x*2): Only optimize if 2 is representable in the
            type.
    
            * gcc.dg/torture/bitint-101.c: New test.
    
    Reviewed-by: Richard Biener <[email protected]>
    (cherry picked from commit d5059b35e03e63c7686288c3554c2922a2c37468)

Diff:
---
 gcc/match.pd                              |  3 ++-
 gcc/testsuite/gcc.dg/torture/bitint-101.c | 27 +++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/gcc/match.pd b/gcc/match.pd
index d1b11483520e..2ab43a773b36 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -4940,7 +4940,8 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
  (plus @0 @0)
  (if (SCALAR_FLOAT_TYPE_P (type))
   (mult @0 { build_real (type, dconst2); })
-  (if (INTEGRAL_TYPE_P (type))
+  (if (INTEGRAL_TYPE_P (type)
+       && TYPE_PRECISION (type) > (2 - TYPE_UNSIGNED (type)))
    (mult @0 { build_int_cst (type, 2); }))))
 
 /* 0 - X  ->  -X.  */
diff --git a/gcc/testsuite/gcc.dg/torture/bitint-101.c b/gcc/testsuite/gcc.dg/torture/bitint-101.c
new file mode 100644
index 000000000000..42f3998ccbfc
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/bitint-101.c
@@ -0,0 +1,27 @@
+/* PR tree-optimization/126257 */
+/* { dg-do run } */
+/* { dg-additional-options "-std=c23" } */
+
+_BitInt(2) a;
+long long b, c, d;
+
+int
+main ()
+{
+  int e;
+  _BitInt(2) f;
+  bool g;
+  long long h = b;
+lab:
+  e = h;
+  if (e != 4)
+    f = -1;
+  f = f + f;
+  a = f;
+  g = d - 4;
+  if (!g)
+    goto lab;
+  c = a;
+  if (c != -2)
+    __builtin_abort ();
+}