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

2 files changed, 8 insertions, 6 deletions

diff --git a/libgo/go/runtime/mpagealloc.go b/libgo/go/runtime/mpagealloc.go
index 2725e3b7c7b..5e40da45d17 100644
--- a/libgo/go/runtime/mpagealloc.go
+++ b/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
 	}
diff --git a/libgo/go/runtime/mpagecache.go b/libgo/go/runtime/mpagecache.go
index 7206e2dbdb7..5bad4f789a1 100644
--- a/libgo/go/runtime/mpagecache.go
+++ b/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)