[PATCH] PR middle-end/122348: ICE in store_constructor from flexible array member

[Roger Sayle]

This patch resolves PR middle-end/122348, an ICE caused by passing a
initialized structure containing a flexible array member by value.
The semantics in C99 (and since gcc 4.4) are that the zero sized array
at the end of the structure is ignored when passing by value.  Hence
for the structure in the PR:

struct S {
    int a;
    int b[];
} s = { 0, { 42 } };

when passed by value, sizeof(s) is considered to be 4 bytes, and on
x86_64 passed in the 32-bit %edi register.  Unfortunately, the code
in store_constructor isn't expecting initialized fields where the
type's DECL_SIZE is NULL, which leads to the ICE.  Fixed by explicitly
ignoring fields where DECL_SIZE is NULL_TREE.  On x86_64, passing "s"
now compiles to just:

f:      xorl    %edi, %edi
        jmp     foo


This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures.  Ok for mainline?

2026-01-25  Roger Sayle  <ro...@nextmovesoftware.com>

gcc/ChangeLog
        * gcc/expr.cc (store_constructor): Ignore fields where DECL_SIZE is
NULL_TREE,
        i.e. flexible array members.

gcc/testsuite/ChangeLog
        * g++.dg/pr122348.C: New C++ testcase.
        * gcc.dg/pr122348.c: New C testcase.


Thanks in advance,
Roger
--


diff --git a/gcc/expr.cc b/gcc/expr.cc
index 70b4eda6df3..b6d593d09a2 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -7579,8 +7579,13 @@ store_constructor (tree exp, rtx target, int cleared, 
poly_int64 size,
            if (cleared && initializer_zerop (value))
              continue;
 
-           if (tree_fits_uhwi_p (DECL_SIZE (field)))
-             bitsize = tree_to_uhwi (DECL_SIZE (field));
+           /* Variable sized arrays are ignored.  */
+           tree decl_size = DECL_SIZE (field);
+           if (!decl_size)
+             continue;
+
+           if (tree_fits_uhwi_p (decl_size))
+             bitsize = tree_to_uhwi (decl_size);
            else
              gcc_unreachable ();
 
diff --git a/gcc/testsuite/g++.dg/pr122348.C b/gcc/testsuite/g++.dg/pr122348.C
new file mode 100644
index 00000000000..4494ef4030d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr122348.C
@@ -0,0 +1,13 @@
+/* middle-end/122348 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+struct S {
+    int a;
+    int b[];
+};
+const struct S s = { 0, { 42 } };
+void foo(struct S arg);
+void f(void) {
+    foo(s);
+}
diff --git a/gcc/testsuite/gcc.dg/pr122348.c b/gcc/testsuite/gcc.dg/pr122348.c
new file mode 100644
index 00000000000..4494ef4030d
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr122348.c
@@ -0,0 +1,13 @@
+/* middle-end/122348 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+struct S {
+    int a;
+    int b[];
+};
+const struct S s = { 0, { 42 } };
+void foo(struct S arg);
+void f(void) {
+    foo(s);
+}