analyzer: stop -ftrivial-auto-var-init from suppressing uninit warnings [PR104270]

GCC 12 has gained two features for dealing with uninitialized variables:

(a) a new -Wanalyzer-use-of-uninitialized-value warning within -fanalyzer
for interprocedural path-sensitive detection of ununit uses, and

(b) a new -ftrivial-auto-var-init option for mitigating some uses of
uninit variables

It turns out that using (b) was thwarting (a), as it led to -fanalyzer
seeing calls to IFN_DEFERRED_INIT, which -fanalyzer wasn't
special-casing, thus treating it as initializing the variables in
question, and thus silencing -Wanalyzer-use-of-uninitialized-value on
them.

invoke.texi says:

"GCC still considers an automatic variable that doesn't have an explicit
initializer as uninitialized, @option{-Wuninitialized} will still report
warning messages on such automatic variables."

and thus -Wanalyzer-use-of-uninitialized-value ought to as well.

This patch adds special-case handling to -fanalyzer for
IFN_DEFERRED_INIT,  so that -fanalyzer will warn on uninit uses of
variables that are mitigated by -ftrivial-auto-var-init.

gcc/analyzer/ChangeLog:
	PR analyzer/104270
	* region-model.cc (region_model::on_call_pre): Handle
	IFN_DEFERRED_INIT.

gcc/testsuite/ChangeLog:
	PR analyzer/104270
	* gcc.dg/analyzer/uninit-trivial-auto-var-init-pattern.c: New
	test.
	* gcc.dg/analyzer/uninit-trivial-auto-var-init-uninitialized.c:
	New test.
	* gcc.dg/analyzer/uninit-trivial-auto-var-init-zero.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

4 files changed, 31 insertions, 0 deletions

diff --git a/gcc/analyzer/region-model.cc b/gcc/analyzer/region-model.cc
index 6810cf5..4c312b0 100644
--- a/gcc/analyzer/region-model.cc
+++ b/gcc/analyzer/region-model.cc
@@ -1109,6 +1109,16 @@ region_model::on_call_pre (const gcall *call, region_model_context *ctxt,
 
   bool unknown_side_effects = false;
 
+  /* Special-case for IFN_DEFERRED_INIT.
+     We want to report uninitialized variables with -fanalyzer (treating
+     -ftrivial-auto-var-init= as purely a mitigation feature).
+     Handle IFN_DEFERRED_INIT by treating it as no-op: don't touch the
+     lhs of the call, so that it is still uninitialized from the point of
+     view of the analyzer.  */
+  if (gimple_call_internal_p (call)
+      && gimple_call_internal_fn (call) == IFN_DEFERRED_INIT)
+    return false;
+
   /* Some of the cases below update the lhs of the call based on the
      return value, but not all.  Provide a default value, which may
      get overwritten below.  */
diff --git a/gcc/testsuite/gcc.dg/analyzer/uninit-trivial-auto-var-init-pattern.c b/gcc/testsuite/gcc.dg/analyzer/uninit-trivial-auto-var-init-pattern.c
new file mode 100644
index 0000000..0b78dc6
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/analyzer/uninit-trivial-auto-var-init-pattern.c
@@ -0,0 +1,7 @@
+/* { dg-additional-options "-ftrivial-auto-var-init=pattern" } */
+
+int test_1 (void)
+{
+  int i; /* { dg-message "region created on stack here" } */
+  return i; /* { dg-warning "use of uninitialized value 'i'" } */
+}
diff --git a/gcc/testsuite/gcc.dg/analyzer/uninit-trivial-auto-var-init-uninitialized.c b/gcc/testsuite/gcc.dg/analyzer/uninit-trivial-auto-var-init-uninitialized.c
new file mode 100644
index 0000000..124d3a3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/analyzer/uninit-trivial-auto-var-init-uninitialized.c
@@ -0,0 +1,7 @@
+/* { dg-additional-options "-ftrivial-auto-var-init=uninitialized" } */
+
+int test_1 (void)
+{
+  int i; /* { dg-message "region created on stack here" } */
+  return i; /* { dg-warning "use of uninitialized value 'i'" } */
+}
diff --git a/gcc/testsuite/gcc.dg/analyzer/uninit-trivial-auto-var-init-zero.c b/gcc/testsuite/gcc.dg/analyzer/uninit-trivial-auto-var-init-zero.c
new file mode 100644
index 0000000..ef7dc67
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/analyzer/uninit-trivial-auto-var-init-zero.c
@@ -0,0 +1,7 @@
+/* { dg-additional-options "-ftrivial-auto-var-init=zero" } */
+
+int test_1 (void)
+{
+  int i; /* { dg-message "region created on stack here" } */
+  return i; /* { dg-warning "use of uninitialized value 'i'" } */
+}