compiler, runtime: optimize zero-sized allocations

Instead of referring to an unused global, use a constant value. This is
safe even when using `-gc=none` (since no actual memory gets allocated)
which wasn't the case before. It should also reduce binary size by a few
bytes for most programs.

compiler/gc.go

@@ -20,6 +20,13 @@ func (b *builder) createAlloc(sizeValue, layoutValue llvm.Value, align int, comm
 		allocFunc = "alloc_noheap"
 	}
 
+	// Allocs that don't allocate anything can return an architecture-specific
+	// sentinel value.
+	if !sizeValue.IsAConstantInt().IsNil() && sizeValue.ZExtValue() == 0 {
+		allocFunc = "alloc_zero"
+	}
+
+	// Make the runtime call.
 	call := b.createRuntimeCall(allocFunc, []llvm.Value{sizeValue, layoutValue}, comment)
 	if align != 0 {
 		// TODO: make sure all callsites set the correct alignment.

compiler/symbol.go

@@ -162,7 +162,7 @@ func (c *compilerContext) getFunction(fn *ssa.Function) (llvm.Type, llvm.Value)
 		llvmFn.AddAttributeAtIndex(1, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0))
 	case "machine.keepAliveNoEscape", "machine.unsafeNoEscape":
 		llvmFn.AddAttributeAtIndex(1, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0))
-	case "runtime.alloc", "runtime.alloc_noheap":
+	case "runtime.alloc", "runtime.alloc_noheap", "runtime.alloc_zero":
 		// Tell the optimizer that runtime.alloc is an allocator, meaning that it
 		// returns values that are never null and never alias to an existing value.
 		for _, attrName := range []string{"noalias", "nonnull"} {

compiler/testdata/gc.ll

@@ -75,7 +75,7 @@ entry:
 define hidden void @main.newStruct(ptr %context) unnamed_addr #1 {
 entry:
   %stackalloc = alloca i8, align 1
-  %new = call align 1 ptr @runtime.alloc(i32 0, ptr nonnull inttoptr (i32 3 to ptr), ptr undef) #3
+  %new = call align 1 ptr @runtime.alloc_zero(i32 0, ptr nonnull inttoptr (i32 3 to ptr), ptr undef) #3
   call void @runtime.trackPointer(ptr nonnull %new, ptr nonnull %stackalloc, ptr undef) #3
   store ptr %new, ptr @main.struct1, align 4
   %new1 = call align 4 dereferenceable(8) ptr @runtime.alloc(i32 8, ptr nonnull inttoptr (i32 3 to ptr), ptr undef) #3
@@ -93,6 +93,9 @@ entry:
   ret void
 }
 
+; Function Attrs: allockind("alloc,zeroed") allocsize(0)
+declare noalias nonnull ptr @runtime.alloc_zero(i32, ptr, ptr) #2
+
 ; Function Attrs: nounwind
 define hidden ptr @main.newFuncValue(ptr %context) unnamed_addr #1 {
 entry: