compiler: revert for package-scope "a = b; b = x" just set "a = x"

Revert CL 245098.  It caused incorrect initialization ordering.

Adjust the runtime package to work even with the CL reverted.

Original description of CL 245098:

    This avoids requiring an init function to initialize the variable.
    This can only be done if x is a static initializer.

    The go1.15rc1 runtime package relies on this optimization.
    The package has a variable "var maxSearchAddr = maxOffAddr".
    The maxSearchAddr variable is used by code that runs before package
    initialization is complete.

For golang/go#51913

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/395994

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-323ab0e6fab89978bdbd83dca9c2ad9c5dcd690f
+62fc90f52da2f52cbe3b4f10e560dc6aa59baeb5
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.cc

@@ -1612,31 +1612,16 @@ Gogo::write_globals()
               // The initializer is constant if it is the zero-value of the
               // variable's type or if the initial value is an immutable value
               // that is not copied to the heap.
-	      Expression* init = var->init();
-
-	      // If we see "a = b; b = x", and x is a static
-	      // initializer, just set a to x.
-	      while (init != NULL && init->var_expression() != NULL)
-		{
-		  Named_object* ino = init->var_expression()->named_object();
-		  if (!ino->is_variable() || ino->package() != NULL)
-		    break;
-		  Expression* ino_init = ino->var_value()->init();
-		  if (ino->var_value()->has_pre_init()
-		      || ino_init == NULL
-		      || !ino_init->is_static_initializer())
-		    break;
-		  init = ino_init;
-		}
-
-              bool is_static_initializer;
-              if (init == NULL)
+              bool is_static_initializer = false;
+              if (var->init() == NULL)
                 is_static_initializer = true;
               else
                 {
                   Type* var_type = var->type();
-                  init = Expression::make_cast(var_type, init, var->location());
-                  is_static_initializer = init->is_static_initializer();
+                  Expression* init = var->init();
+                  Expression* init_cast =
+                      Expression::make_cast(var_type, init, var->location());
+                  is_static_initializer = init_cast->is_static_initializer();
                 }
 
 	      // Non-constant variable initializations might need to create
@@ -1655,15 +1640,7 @@ Gogo::write_globals()
                     }
 		  var_init_fn = init_fndecl;
 		}
-
-	      Bexpression* var_binit;
-	      if (init == NULL)
-		var_binit = NULL;
-	      else
-		{
-		  Translate_context context(this, var_init_fn, NULL, NULL);
-		  var_binit = init->get_backend(&context);
-		}
+              Bexpression* var_binit = var->get_init(this, var_init_fn);
 
               if (var_binit == NULL)
 		;

libgo/go/runtime/mpagealloc.go

@@ -87,7 +87,9 @@ const (
 //
 // We alias maxOffAddr just to make it clear that this is the maximum address
 // for the page allocator's search space. See maxOffAddr for details.
-var maxSearchAddr = maxOffAddr
+func maxSearchAddr() offAddr {
+	return maxOffAddr
+}
 
 // Global chunk index.
 //
@@ -331,13 +333,13 @@ func (p *pageAlloc) init(mheapLock *mutex, sysStat *sysMemStat) {
 	p.sysInit()
 
 	// Start with the searchAddr in a state indicating there's no free memory.
-	p.searchAddr = maxSearchAddr
+	p.searchAddr = maxSearchAddr()
 
 	// Set the mheapLock.
 	p.mheapLock = mheapLock
 
 	// Initialize scavenge tracking state.
-	p.scav.scavLWM = maxSearchAddr
+	p.scav.scavLWM = maxSearchAddr()
 }
 
 // tryChunkOf returns the bitmap data for the given chunk.
@@ -760,7 +762,7 @@ nextLevel:
 		}
 		if l == 0 {
 			// We're at level zero, so that means we've exhausted our search.
-			return 0, maxSearchAddr
+			return 0, maxSearchAddr()
 		}
 
 		// We're not at level zero, and we exhausted the level we were looking in.
@@ -854,7 +856,7 @@ func (p *pageAlloc) alloc(npages uintptr) (addr uintptr, scav uintptr) {
 			// exhausted. Otherwise, the heap still might have free
 			// space in it, just not enough contiguous space to
 			// accommodate npages.
-			p.searchAddr = maxSearchAddr
+			p.searchAddr = maxSearchAddr()
 		}
 		return 0, 0
 	}

libgo/go/runtime/mpagecache.go

@@ -143,7 +143,7 @@ func (p *pageAlloc) allocToCache() pageCache {
 		if addr == 0 {
 			// We failed to find adequate free space, so mark the searchAddr as OoM
 			// and return an empty pageCache.
-			p.searchAddr = maxSearchAddr
+			p.searchAddr = maxSearchAddr()
 			return pageCache{}
 		}
 		ci := chunkIndex(addr)