[AsmParser][ARM] Make .thumb_func imply .thumb

GNU as documentation states that a `.thumb_func` directive implies `.thumb`, teach the asm parser to switch mode whenever it's encountered. On the other hand the labeled form, exclusive to Apple's toolchain, doesn't switch mode at all.

Reviewed By: nickdesaulniers, peter.smith

Differential Revision: https://reviews.llvm.org/D101975

lib/Target/ARM/AsmParser/ARMAsmParser.cpp

@@ -11165,6 +11165,12 @@ bool ARMAsmParser::parseDirectiveThumbFunc(SMLoc L) {
                  "unexpected token in '.thumb_func' directive"))
     return true;
 
+  // .thumb_func implies .thumb
+  if (!isThumb())
+    SwitchMode();
+
+  getParser().getStreamer().emitAssemblerFlag(MCAF_Code16);
+
   NextSymbolIsThumb = true;
   return false;
 }

test/MC/ARM/thumb_func-implies-thumb.s

@@ -0,0 +1,31 @@
+@ RUN: llvm-mc -triple=armv7-darwin- -show-encoding < %s | FileCheck %s
+.syntax unified
+
+.text
+
+.arm
+@ Ensure the plain form switches mode.
+.thumb_func
+@ CHECK: .code 16
+@ CHECK-LABEL: foo
+foo:
+    bx lr
+
+.arm
+@ Ensure the labeled form doesn't switch mode.
+.thumb_func bar
+@ CHECK-NOT: .code 16
+@ CHECK-LABEL: bar
+bar:
+    bx lr
+
+.arm
+@ Ensure the nop is assembled in thumb mode, even though the baz symbol is
+@ defined later.
+.thumb_func
+nop
+@ CHECK: .code 16
+@ CHECK-NEXT: nop
+@ CHECK-LABEL: baz
+baz:
+    bx lr