[Clang] Add check for -mstack-alignment (llvm#143124)

Currently the assertion in Alignment.h is triggered if a wrong value is
passed -mstack-alignment option:
```
Assertion `(Value == 0 || llvm::isPowerOf2_64(Value)) && "Alignment is neither 0 nor
a power of 2"' failed.
```

Added check in clang driver for the value of -mstack-alignment option,
and emitted an error message when the wrong value was passed.

Author: fzou1

clang/lib/Driver/ToolChains/Clang.cpp

@@ -6907,9 +6907,17 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   Args.addOptInFlag(CmdArgs, options::OPT_mstackrealign,
                     options::OPT_mno_stackrealign);
 
-  if (Args.hasArg(options::OPT_mstack_alignment)) {
-    StringRef alignment = Args.getLastArgValue(options::OPT_mstack_alignment);
-    CmdArgs.push_back(Args.MakeArgString("-mstack-alignment=" + alignment));
+  if (const Arg *A = Args.getLastArg(options::OPT_mstack_alignment)) {
+    StringRef Value = A->getValue();
+    int64_t Alignment = 0;
+    if (Value.getAsInteger(10, Alignment) || Alignment < 0)
+      D.Diag(diag::err_drv_invalid_argument_to_option)
+          << Value << A->getOption().getName();
+    else if (Alignment & (Alignment - 1))
+      D.Diag(diag::err_drv_alignment_not_power_of_two)
+          << A->getAsString(Args) << Value;
+    else
+      CmdArgs.push_back(Args.MakeArgString("-mstack-alignment=" + Value));
   }
 
   if (Args.hasArg(options::OPT_mstack_probe_size)) {

clang/test/Driver/stack-alignment.c

@@ -0,0 +1,11 @@
+// RUN: not %clang -### -mstack-alignment=-1 %s 2>&1 | FileCheck %s --check-prefix=CHECK_NEG_1
+// RUN: %clang -### -mstack-alignment=0 %s 2>&1 | FileCheck %s --check-prefix=CHECK_0
+// RUN: %clang -### -mstack-alignment=1 %s 2>&1 | FileCheck %s --check-prefix=CHECK_1
+// RUN: %clang -### -mstack-alignment=4 %s 2>&1 | FileCheck %s --check-prefix=CHECK_4
+// RUN: not %clang -### -mstack-alignment=5 %s 2>&1 | FileCheck %s --check-prefix=CHECK_5
+
+// CHECK_NEG_1: error: invalid argument '-1' to -mstack-alignment=
+// CHECK_0: -mstack-alignment=0
+// CHECK_1: -mstack-alignment=1
+// CHECK_4: -mstack-alignment=4
+// CHECK_5: error: alignment is not a power of 2 in '-mstack-alignment=5'