test: nlist: make resilient against -Wl,--gc-sections

With linker sections GC enabled, we get a test failure in `nlist.c`:
```
nlist: nlist.c:72: main: Assertion `rc == 0' failed.
```

This turns out to be because several sections used by the test can be discarded:
```
ld: removing unused section '.text.func_pub' in file 'nlist.o'
ld: removing unused section '.bss.data_pub_uninit' in file 'nlist.o'
ld: removing unused section '.data.data_pub_init' in file 'nlist.o'
```

Reproduced with `CFLAGS="-Og -fdata-sections -ffunction-sections"` and
`LDFLAGS="-Wl,-O1 -Wl,--as-needed -Wl,--gc-sections -Wl,-z,start-stop-gc"`.

Additionally, `LDFLAGS="... -Wl,--print-gc-sections"` can help with diagnosing
which sections get removed.

We already mark these symbols as `used`, but we need `retain` [0] for them
to survive linker GC too.

[0] https://releases.llvm.org/18.1.0/tools/lld/docs/ELF/start-stop-gc.html#annotate-c-identifier-name-sections

Closes: !29
Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Guillem Jover <guillem@hadrons.org>

diff --git a/test/nlist.c b/test/nlist.c
index 558524f1ee4bc66a2720cfc6660c368676e6525a..0031a4dac389b4c521e24776eff17034dc31473d 100644 (file)
--- a/test/nlist.c
+++ b/test/nlist.c
@@ -33,17 +33,29 @@
 #include <fcntl.h>
 #include <nlist.h>
 
+#if defined(__has_attribute)
+#if __has_attribute(__retain__)
+#define ATTRIBUTE_RETAIN __attribute__((__retain__))
+#else
+#define ATTRIBUTE_RETAIN
+#endif
+#endif
+
 static int data_prv_init = 50;
 extern int data_pub_init;
 extern int data_pub_uninit[2048];
 extern int *data_pub_ptr;
 
+/* GCC's __has_attribute has odd behavior for retain: https://gcc.gnu.org/PR99587 */
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wattributes"
 int *data_pub_ptr = &data_prv_init;
-int data_pub_init __attribute__((__used__)) = 10;
-int data_pub_uninit[2048] __attribute__((__used__));
+int data_pub_init ATTRIBUTE_RETAIN __attribute__((__used__)) = 10;
+int data_pub_uninit[2048] ATTRIBUTE_RETAIN __attribute__((__used__));
 
 extern int
-func_pub(void) __attribute__((__used__)) ;
+func_pub(void) ATTRIBUTE_RETAIN __attribute__((__used__));
+#pragma GCC diagnostic pop
 
 int
 func_pub(void)