[gcc r11-3242] analyzer: getchar has no side-effects

[gcc r11-3242] analyzer: getchar has no side-effects

[David Malcolm]

https://gcc.gnu.org/g:e097c9ab83192fc2f738ec6426a275282e9a51ea

commit r11-3242-ge097c9ab83192fc2f738ec6426a275282e9a51ea
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Wed Sep 16 13:12:39 2020 -0400

    analyzer: getchar has no side-effects
    
    Seen whilst debugging another issue, where the analyzer was assuming
    conservatively that a call to getchar could clobber a global.
    
    This is handled for most of the other stdio functions by the list
    in sm-file.cc
    
    gcc/analyzer/ChangeLog:
            * region-model.cc (region_model::on_call_pre): Treat getchar as
            having no side-effects.
    
    gcc/testsuite/ChangeLog:
            * gcc.dg/analyzer/getchar-1.c: New test.

Diff:
---
 gcc/analyzer/region-model.cc              |  5 +++++
 gcc/testsuite/gcc.dg/analyzer/getchar-1.c | 19 +++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/gcc/analyzer/region-model.cc b/gcc/analyzer/region-model.cc
index d53272e4332..1312391557d 100644
--- a/gcc/analyzer/region-model.cc
+++ b/gcc/analyzer/region-model.cc
@@ -732,6 +732,11 @@ region_model::on_call_pre (const gcall *call, region_model_context *ctxt)
 	return impl_call_calloc (cd);
       else if (is_named_call_p (callee_fndecl, "alloca", call, 1))
 	return impl_call_alloca (cd);
+      else if (is_named_call_p (callee_fndecl, "getchar", call, 0))
+	{
+	  /* No side-effects (tracking stream state is out-of-scope
+	     for the analyzer).  */
+	}
       else if (is_named_call_p (callee_fndecl, "memset", call, 3))
 	{
 	  impl_call_memset (cd);
diff --git a/gcc/testsuite/gcc.dg/analyzer/getchar-1.c b/gcc/testsuite/gcc.dg/analyzer/getchar-1.c
new file mode 100644
index 00000000000..25595e0786e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/analyzer/getchar-1.c
@@ -0,0 +1,19 @@
+#include <stdio.h>
+#include "analyzer-decls.h"
+
+int test_1 (void)
+{
+  int c = getchar ();
+  return c;
+}
+
+int glob_2;
+int test_2 (void)
+{
+  int c;
+  glob_2 = 42;
+  __analyzer_eval (glob_2 == 42); /* { dg-warning "TRUE" } */
+  c = getchar ();
+  __analyzer_eval (glob_2 == 42); /* { dg-warning "TRUE" } */
+  return c;
+}