compiler: mark stringFromBytes as nocapture/readonly to help escape analysis

Fixes #4525

Author: dgryski

compiler/symbol.go

@@ -172,6 +172,9 @@ func (c *compilerContext) getFunction(fn *ssa.Function) (llvm.Type, llvm.Value)
 		llvmFn.AddAttributeAtIndex(1, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0))
 		llvmFn.AddAttributeAtIndex(2, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("readonly"), 0))
 		llvmFn.AddAttributeAtIndex(2, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0))
+	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.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

@@ -49,6 +49,11 @@ func main() {
 		n4 = n5
 	}()
 	println(n4, n5)
+
+	// This shouldn't escape.
+	var buf [32]byte
+	s := string(buf[:])
+	println(len(s))
 }
 
 func derefInt(x *int) int {