compiler: add optsize function attr to reduce binary size

This is also added by Clang at -Oz and results in slightly smaller
binaries.

compiler/optimizer.go

@@ -51,6 +51,16 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) erro
 		return errors.New("optimizations caused a verification failure")
 	}
 
+	if sizeLevel >= 2 {
+		// Set the "optsize" attribute to make slightly smaller binaries at the
+		// cost of some performance.
+		kind := llvm.AttributeKindID("optsize")
+		attr := c.ctx.CreateEnumAttribute(kind, 0)
+		for fn := c.mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
+			fn.AddFunctionAttr(attr)
+		}
+	}
+
 	// Run module passes.
 	modPasses := llvm.NewPassManager()
 	defer modPasses.Dispose()