code review 11676043: runtime: reduce frame size for runtime.cgocallback_gofunc

src/pkg/runtime/cgocall.c

 // Copyright 2009 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 #include "runtime.h"
 #include "arch_GOARCH.h"
 #include "stack.h"
 #include "cgocall.h"
 #include "race.h"
 
@@ skipping 218 matching lines @@
 static FuncVal unwindmf = {unwindm};
 
 typedef struct CallbackArgs CallbackArgs;
 struct CallbackArgs
 {
         FuncVal *fn;
         void *arg;
         uintptr argsize;
 };
 
+#define CBARGS (CallbackArgs*)((byte*)m->g0->sched.sp+(3+(thechar=='5'))*sizeof(void*))
+
 void
 runtime·cgocallbackg(void)
 {
         Defer d;
         CallbackArgs *cb;
 
         if(m->racecall) {
-                cb = (CallbackArgs*)((byte*)m->g0->sched.sp+3*sizeof(void*));
+                cb = CBARGS;
                 reflect·call(cb->fn, cb->arg, cb->argsize);
                 return;
         }
 
         if(g != m->curg)
                 runtime·throw("runtime: bad g in cgocallback");
 
         runtime·exitsyscall();  // coming out of cgo call
 
         if(m->needextram) {
                 m->needextram = 0;
                 runtime·newextram();
         }
 
         // Add entry to defer stack in case of panic.
         d.fn = &unwindmf;
         d.siz = 0;
         d.link = g->defer;
         d.argp = (void*)-1;  // unused because unwindm never recovers
         d.special = true;
         d.free = false;
         g->defer = &d;
 
         if(raceenabled)
                 runtime·raceacquire(&cgosync);
 
         // Invoke callback.
-        cb = (CallbackArgs*)((byte*)m->g0->sched.sp+3*sizeof(void*));
+        cb = CBARGS;
         reflect·call(cb->fn, cb->arg, cb->argsize);
 
         if(raceenabled)
                 runtime·racereleasemerge(&cgosync);
 
         // Pop defer.
         // Do not unwind m->g0->sched.sp.
         // Our caller, cgocallback, will do that.
         if(g->defer != &d || d.fn != &unwindmf)
                 runtime·throw("runtime: bad defer entry in cgocallback");
@@ skipping 30 matching lines @@
 runtime·cgounimpl(void) // called from (incomplete) assembly
 {
         runtime·throw("runtime: cgo not implemented");
 }
 
 // For cgo-using programs with external linking,
 // export "main" (defined in assembly) so that libc can handle basic
 // C runtime startup and call the Go program as if it were
 // the C main function.
 #pragma cgo_export_static main