[gcc r15-10673] analyzer: fold X + (-X) to zero [PR122975]

[David Malcolm]

https://gcc.gnu.org/g:e869244a134c84ef9f5ba15d8005bf66f9f2048c

commit r15-10673-ge869244a134c84ef9f5ba15d8005bf66f9f2048c
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Mon Dec 15 11:48:49 2025 -0500

    analyzer: fold X + (-X) to zero [PR122975]
    
    gcc/analyzer/ChangeLog:
            PR analyzer/122975
            * region-model-manager.cc (region_model_manager::maybe_fold_binop): Fold
            X + (-X) to zero.
    
    gcc/testsuite/ChangeLog:
            PR analyzer/122975
            * c-c++-common/analyzer/arith-1.c: New test.
            * c-c++-common/analyzer/infinite-recursion-pr122975.c: New test.
    
    Signed-off-by: David Malcolm <dmalcolm@redhat.com>
    
    (cherry picked from commit c48970fc4a4d1747a50e6abd1634757212420c04)

Diff:
---
 gcc/analyzer/region-model-manager.cc                        |  5 +++++
 gcc/testsuite/c-c++-common/analyzer/arith-1.c               | 13 +++++++++++++
 .../c-c++-common/analyzer/infinite-recursion-pr122975.c     |  5 +++++
 3 files changed, 23 insertions(+)

diff --git a/gcc/analyzer/region-model-manager.cc b/gcc/analyzer/region-model-manager.cc
index dfce420065d4..b5722f565acd 100644
--- a/gcc/analyzer/region-model-manager.cc
+++ b/gcc/analyzer/region-model-manager.cc
@@ -698,6 +698,11 @@ region_model_manager::maybe_fold_binop (tree type, enum tree_code op,
       /* (VAL + 0) -> VAL.  */
       if (cst1 && zerop (cst1))
 	return get_or_create_cast (type, arg0);
+      /* X + (-X) -> 0.  */
+      if (const unaryop_svalue *unary_op = arg1->dyn_cast_unaryop_svalue ())
+	if (unary_op->get_op () == NEGATE_EXPR
+	    && unary_op->get_arg () == arg0)
+	  return get_or_create_int_cst (type, 0);
       break;
     case MINUS_EXPR:
       /* (VAL - 0) -> VAL.  */
diff --git a/gcc/testsuite/c-c++-common/analyzer/arith-1.c b/gcc/testsuite/c-c++-common/analyzer/arith-1.c
new file mode 100644
index 000000000000..af58466c7bc7
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/analyzer/arith-1.c
@@ -0,0 +1,13 @@
+#include "analyzer-decls.h"
+
+static int __attribute__((noipa))
+negate_int (int x)
+{
+  return -x;
+}
+
+void
+test_1 (int a)
+{
+  __analyzer_eval (a + negate_int (a) == 0); /* { dg-warning "TRUE" } */
+}
diff --git a/gcc/testsuite/c-c++-common/analyzer/infinite-recursion-pr122975.c b/gcc/testsuite/c-c++-common/analyzer/infinite-recursion-pr122975.c
new file mode 100644
index 000000000000..c241f4169f7f
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/analyzer/infinite-recursion-pr122975.c
@@ -0,0 +1,5 @@
+void fun(int x, int y)
+{
+  if (x + y != 0) 
+    fun(x, -x); /* { dg-bogus "infinite recursion" } */
+}