Take a simpler approach to gcc warnings in IOManager.c

We have lots of functions with conditional implementations for
different I/O managers. Some functions, for some I/O managers,
naturally have implementations that do nothing or barf. When only one
such I/O manager is enabled then the whole function implementation will
have an implementation that does nothing or barfs. This then results in
warnings from gcc that parameters are unused, or that the function
should be marked with attribute noreturn (since barf does not return).
The USED_IF_THREADS trick for fine-grained warning supression is fine
for just two cases, but an equivalent here would need
USED_IF_THE_ONLY_ENABLED_IOMGR_IS_X_OR_Y which would have combinitorial
blowup. So we take a coarse grained approach and simply disable these
two warnings for the whole file.

So we use a GCC pragma, with its handy push/pop support:

 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wsuggest-attribute=noreturn"
 #pragma GCC diagnostic ignored "-Wunused-parameter"

...

 #pragma GCC diagnostic pop

rts/IOManager.c

@@ -45,6 +45,24 @@
 #include <string.h>
 
 
+/* We have lots of functions below with conditional implementations for
+ * different I/O managers. Some functions, for some I/O managers, naturally
+ * have implementations that do nothing or barf. When only one such I/O
+ * manager is enabled then the whole function implementation will have an
+ * implementation that does nothing or barfs. This then results in warnings
+ * from gcc that parameters are unused, or that the function should be marked
+ * with attribute noreturn (since barf does not return). The USED_IF_THREADS
+ * trick for fine-grained warning supression is fine for just two cases, but an
+ * equivalent here would need USED_IF_THE_ONLY_ENABLED_IOMGR_IS_X_OR_Y which
+ * would have combinitorial blowup. So we take a coarse grained approach and
+ * simply disable these two warnings for the whole file.
+ */
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wsuggest-attribute=noreturn"
+#pragma GCC diagnostic ignored "-Wmissing-noreturn"
+#pragma GCC diagnostic ignored "-Wunused-parameter"
+
+
 /* Global var to tell us which I/O manager impl we are using */
 IOManagerType iomgr_type;
 
@@ -305,7 +323,7 @@ initIOManager(void)
 /* Called from forkProcess in the child process on the surviving capability.
  */
 void
-initIOManagerAfterFork(Capability **pcap USED_IF_THREADS_AND_NOT_MINGW32)
+initIOManagerAfterFork(Capability **pcap)
 {
 
     switch (iomgr_type) {
@@ -370,7 +388,7 @@ stopIOManager(void)
 /* Called in the RTS shutdown after the scheduler exits
  */
 void
-exitIOManager(bool wait_threads USED_IF_NOT_THREADS_AND_MINGW32)
+exitIOManager(bool wait_threads)
 {
     switch (iomgr_type) {
 #if defined(IOMGR_ENABLED_WINIO)
@@ -424,9 +442,9 @@ void wakeupIOManager(void)
     }
 }
 
-void markCapabilityIOManager(evac_fn       evac  USED_IF_NOT_THREADS,
-                             void         *user  USED_IF_NOT_THREADS,
-                             CapIOManager *iomgr USED_IF_NOT_THREADS)
+void markCapabilityIOManager(evac_fn       evac,
+                             void         *user,
+                             CapIOManager *iomgr)
 {
 
     switch (iomgr_type) {
@@ -456,7 +474,7 @@ void markCapabilityIOManager(evac_fn       evac  USED_IF_NOT_THREADS,
  */
 #if !defined(mingw32_HOST_OS)
 void
-setIOManagerControlFd(uint32_t cap_no USED_IF_THREADS, int fd USED_IF_THREADS) {
+setIOManagerControlFd(uint32_t cap_no, int fd) {
 #if defined(THREADED_RTS)
     if (cap_no < getNumCapabilities()) {
         RELAXED_STORE(&getCapability(cap_no)->iomgr->control_fd, fd);
@@ -468,7 +486,7 @@ setIOManagerControlFd(uint32_t cap_no USED_IF_THREADS, int fd USED_IF_THREADS) {
 #endif
 
 
-bool anyPendingTimeoutsOrIO(CapIOManager *iomgr USED_IF_NOT_THREADS)
+bool anyPendingTimeoutsOrIO(CapIOManager *iomgr)
 {
     switch (iomgr_type) {
 #if defined(IOMGR_ENABLED_SELECT)
@@ -520,13 +538,11 @@ bool anyPendingTimeoutsOrIO(CapIOManager *iomgr USED_IF_NOT_THREADS)
     }
 }
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wsuggest-attribute=noreturn"
 
-void syncIOWaitReady(Capability   *cap USED_IF_NOT_THREADS,
-                     StgTSO       *tso USED_IF_NOT_THREADS,
-                     IOReadOrWrite rw  USED_IF_NOT_THREADS,
-                     HsInt         fd  USED_IF_NOT_THREADS)
+void syncIOWaitReady(Capability   *cap,
+                     StgTSO       *tso,
+                     IOReadOrWrite rw,
+                     HsInt         fd)
 {
     ASSERT(tso->why_blocked == NotBlocked);
     switch (iomgr_type) {
@@ -600,7 +616,6 @@ void syncDelay(Capability *cap, StgTSO *tso, HsInt us_delay)
             barf("syncDelay not supported for I/O manager %d", iomgr_type);
     }
 }
-#pragma GCC diagnostic pop
 
 #if defined(IOMGR_ENABLED_SELECT) || defined(IOMGR_ENABLED_WIN32_LEGACY)
 void appendToIOBlockedQueue(Capability *cap, StgTSO *tso)
@@ -658,3 +673,7 @@ bool is_io_mng_native_p (void)
             return false;
     }
 }
+
+
+/* See comment above with the #pragma GCC diagnostic push */
+#pragma GCC diagnostic pop

rts/IOManager.h

@@ -317,19 +317,4 @@ bool anyPendingTimeoutsOrIO(CapIOManager *iomgr);
 void awaitEvent(Capability *cap, bool wait);
 #endif
 
-
-/* Pedantic warning cleanliness
- */
-#if !defined(THREADED_RTS) && defined(mingw32_HOST_OS)
-#define USED_IF_NOT_THREADS_AND_MINGW32
-#else
-#define USED_IF_NOT_THREADS_AND_MINGW32 STG_UNUSED
-#endif
-
-#if defined(THREADED_RTS) && !defined(mingw32_HOST_OS)
-#define USED_IF_THREADS_AND_NOT_MINGW32
-#else
-#define USED_IF_THREADS_AND_NOT_MINGW32 STG_UNUSED
-#endif
-
 #include "EndPrivate.h"