compiler: mark stringFromRunes as nocapture/readonly

Author: dgryski

compiler/symbol.go

@@ -175,6 +175,9 @@ func (c *compilerContext) getFunction(fn *ssa.Function) (llvm.Type, llvm.Value)
 	case "runtime.stringFromBytes":
 		llvmFn.AddAttributeAtIndex(1, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0))
 		llvmFn.AddAttributeAtIndex(1, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("readonly"), 0))
+	case "runtime.stringFromRunes":
+		llvmFn.AddAttributeAtIndex(1, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0))
+		llvmFn.AddAttributeAtIndex(1, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("readonly"), 0))
 	case "runtime.trackPointer":
 		// This function is necessary for tracking pointers on the stack in a
 		// portable way (see gc_stack_portable.go). Indicate to the optimizer

transform/testdata/allocs2.go

@@ -54,6 +54,10 @@ func main() {
 	var buf [32]byte
 	s := string(buf[:])
 	println(len(s))
+
+	var rbuf [5]rune
+	s = string(rbuf[:])
+	println(s)
 }
 
 func derefInt(x *int) int {