hw/display/qxl: Suppress clang-7 warning about misaligned atomic operation

If QEMU is compiled with clang-7 it results in the warning:

hw/display/qxl.c:1884:19: error: misaligned or large atomic operation
may incur significant performance penalty [-Werror,-Watomic-alignment]
    old_pending = atomic_fetch_or(&d->ram->int_pending, le_events);
                  ^

This is because the Spice headers forgot to define the QXLRam struct
with the '__aligned__(4)' attribute.  clang 7 and newer will thus
warn that the access here to int_pending might not be 4-aligned
(because the QXLRam object d->ram points at might start at a
misaligned address).  In fact we set up d->ram in init_qxl_ram() so
it always starts at a 4K boundary, so we know the atomic access here
is OK.

Newer Spice versions (with Spice commit
beda5ec7a6848be20c0cac2a9a8ef2a41e8069c1) will fix the bug;
for older Spice versions, work around it by telling the compiler
explicitly that the alignment is OK using __builtin_assume_aligned().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180927155538.699-1-peter.maydell@linaro.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
index 5843812710..bf47e7bee4 100644
--- a/include/qemu/compiler.h
+++ b/include/qemu/compiler.h
@@ -122,6 +122,15 @@
 #ifndef __has_feature
 #define __has_feature(x) 0 /* compatibility with non-clang compilers */
 #endif
+
+#ifndef __has_builtin
+#define __has_builtin(x) 0 /* compatibility with non-clang compilers */
+#endif
+
+#if __has_builtin(__builtin_assume_aligned) || QEMU_GNUC_PREREQ(4, 7)
+#define HAS_ASSUME_ALIGNED
+#endif
+
 /* Implement C11 _Generic via GCC builtins.  Example:
  *
  *    QEMU_GENERIC(x, (float, sinf), (long double, sinl), sin) (x)