compiler: add minsize attribute for -Oz

This matches the behavior of Clang, which uses optsize for -Os and adds
minsize for -Oz.

The code size change is all over the map, but using a hacked together
size comparison tool I've found that there is a slight reduction in
binary size overall (-1.6% with the tinygo smoke tests and -0.8% for the
drivers smoke test).

compiler/symbol.go

@@ -327,13 +327,20 @@ func getParams(sig *types.Signature) []*types.Var {
 // addStandardDeclaredAttributes adds attributes that are set for any function,
 // whether declared or defined.
 func (c *compilerContext) addStandardDeclaredAttributes(llvmFn llvm.Value) {
-	if c.SizeLevel >= 2 {
+	if c.SizeLevel >= 1 {
 		// Set the "optsize" attribute to make slightly smaller binaries at the
-		// cost of some performance.
+		// cost of minimal performance loss (-Os in Clang).
 		kind := llvm.AttributeKindID("optsize")
 		attr := c.ctx.CreateEnumAttribute(kind, 0)
 		llvmFn.AddFunctionAttr(attr)
 	}
+	if c.SizeLevel >= 2 {
+		// Set the "minsize" attribute to reduce code size even further,
+		// regardless of performance loss (-Oz in Clang).
+		kind := llvm.AttributeKindID("minsize")
+		attr := c.ctx.CreateEnumAttribute(kind, 0)
+		llvmFn.AddFunctionAttr(attr)
+	}
 }
 
 // addStandardDefinedAttributes adds the set of attributes that are added to

stacksize/stacksize.go

@@ -180,8 +180,8 @@ func CallGraph(f *elf.File, callsIndirectFunction []string) (map[string][]*CallN
 					// used for getting a function pointer
 					isCall = false
 				case elf.R_ARM_ABS32:
-					// used in the reset vector for pointers
+					// when compiling with -Oz (minsize), used for calling
-					isCall = false
+					isCall = true
 				default:
 					return nil, fmt.Errorf("unknown relocation: %s", relocType)
 				}

transform/transform.go

@@ -22,7 +22,10 @@ import (
 // the -opt= compiler flag.
 func AddStandardAttributes(fn llvm.Value, config *compileopts.Config) {
 	_, sizeLevel, _ := config.OptLevels()
-	if sizeLevel >= 2 {
+	if sizeLevel >= 1 {
 		fn.AddFunctionAttr(fn.Type().Context().CreateEnumAttribute(llvm.AttributeKindID("optsize"), 0))
 	}
+	if sizeLevel >= 2 {
+		fn.AddFunctionAttr(fn.Type().Context().CreateEnumAttribute(llvm.AttributeKindID("minsize"), 0))
+	}
 }