misc: Update attribute syntax, and reorganize compiler.hh.

This change replaces the __attribute__ syntax with the now standard [[]]
syntax. It also reorganizes compiler.hh so that all special macros have
some explanatory text saying what they do, and each attribute which has a
standard version can use that if available and what version of c++ it's
standard in is put in a comment.

Also, the requirements as far as where you put [[]] style attributes are
a little more strict than the old school __attribute__ style. The use of
the attribute macros was updated to fit these new, more strict
requirements.

Change-Id: Iace44306a534111f1c38b9856dc9e88cd9b49d2a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35219
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/virtio/base.hh

@@ -451,10 +451,10 @@ public:
         typedef uint16_t Flags;
         typedef uint16_t Index;
 
-        struct Header {
+        struct M5_ATTR_PACKED Header {
             Flags flags;
             Index index;
-        } M5_ATTR_PACKED;
+        };
 
         VirtRing<T>(PortProxy &proxy, ByteOrder bo, uint16_t size) :
             header{0, 0}, ring(size), _proxy(proxy), _base(0), byteOrder(bo)

src/dev/virtio/block.hh

@@ -81,9 +81,9 @@ class VirtIOBlock : public VirtIODeviceBase
      * @note This needs to be changed if the supported feature set
      * changes!
      */
-    struct Config {
+    struct M5_ATTR_PACKED Config {
         uint64_t capacity;
-    } M5_ATTR_PACKED;
+    };
     Config config;
 
     /** @{
@@ -122,11 +122,11 @@ class VirtIOBlock : public VirtIODeviceBase
     /** @} */
 
     /** VirtIO block device request as sent by guest */
-    struct BlkRequest {
+    struct M5_ATTR_PACKED BlkRequest {
         RequestType type;
         uint32_t reserved;
         uint64_t sector;
-    } M5_ATTR_PACKED;
+    };
 
     /**
      * Device read request.

src/dev/virtio/console.hh

@@ -77,10 +77,10 @@ class VirtIOConsole : public VirtIODeviceBase
      * @note This needs to be changed if the multiport feature is
      * announced!
      */
-    struct Config {
+    struct M5_ATTR_PACKED Config {
         uint16_t cols;
         uint16_t rows;
-    } M5_ATTR_PACKED;
+    };
 
     /** Currently active configuration (host byte order) */
     Config config;

src/dev/virtio/fs9p.hh

@@ -50,14 +50,14 @@ struct VirtIO9PBaseParams;
 typedef uint8_t P9MsgType;
 typedef uint16_t P9Tag;
 
-struct P9MsgHeader {
+struct M5_ATTR_PACKED P9MsgHeader {
     /** Length including header */
     uint32_t len;
     /** Message type */
     P9MsgType type;
     /** Message tag */
     P9Tag tag;
-} M5_ATTR_PACKED;
+};
 
 /** Convert p9 byte order (LE) to host byte order */
 template <typename T> inline T
@@ -120,10 +120,10 @@ class VirtIO9PBase : public VirtIODeviceBase
      * @note The fields in this structure depend on the features
      * exposed to the guest.
      */
-    struct Config {
+    struct M5_ATTR_PACKED Config {
         uint16_t len;
         char tag[];
-    } M5_ATTR_PACKED;
+    };
 
     /** Currently active configuration (host byte order) */
     std::unique_ptr<Config> config;

src/dev/virtio/pci.cc

@@ -65,7 +65,7 @@ PciVirtIO::~PciVirtIO()
 Tick
 PciVirtIO::read(PacketPtr pkt)
 {
-    const unsigned M5_VAR_USED size(pkt->getSize());
+    M5_VAR_USED const unsigned size(pkt->getSize());
     int bar;
     Addr offset;
     if (!getBAR(pkt->getAddr(), bar, offset))
@@ -146,7 +146,7 @@ PciVirtIO::read(PacketPtr pkt)
 Tick
 PciVirtIO::write(PacketPtr pkt)
 {
-    const unsigned M5_VAR_USED size(pkt->getSize());
+    M5_VAR_USED const unsigned size(pkt->getSize());
     int bar;
     Addr offset;
     if (!getBAR(pkt->getAddr(), bar, offset))