Do not use frame pointer by default for MSP430

[dmikushin]

Summary

3.3.9 Frame Pointer
MSP430 does not use a frame pointer. This effectively limits a single call frame to 0x7fff bytes, which is the minimum SP offset supported by any instruction

We do allow user to manually enable frame pointer. GCC toolchain uses the same behavior.

[grimar]

Testcase?

[dmikushin]

@grimar Please find the updated patch with a test case.

[grimar]

Thanks for adding a test case! I know not enough about msp430/clang to verify it, unfortunately.
(I just asked for a test because it is always useful to have one for any non-NFC patch).

[asl]

Patch fails to apply. Please rebase.

[krisb]

test/CodeGen/msp430-fp-elim.c

17

This test as it is will fail after integrated assembler will be turned on by default (see https://reviews.llvm.org/D56787). Since calla instruction isn't supported yet, the assembler will report an error of 'invalid instruction mnemonic'.
So, please, choose another instruction from ones that are already supported. Or turn -integrated-as off.

[dmikushin]

Unbreaking clang driver patch. Not using calla asm instruction, as recomended.

[asl]

Ok now

lib/Driver/ToolChains/Clang.cpp

 }
 
 static bool useFramePointerForTargetByDefault(const ArgList &Args,
                                               const llvm::Triple &Triple) {
   switch (Triple.getArch()) {
   case llvm::Triple::xcore:
   case llvm::Triple::wasm32:
   case llvm::Triple::wasm64:
+  case llvm::Triple::msp430:
     // XCore never wants frame pointers, regardless of OS.
     // WebAssembly never wants frame pointers.
     return false;
   case llvm::Triple::riscv32:
   case llvm::Triple::riscv64:
     return !areOptimizationsEnabled(Args);
   default:
     break;

test/CodeGen/msp430-fp-elim.c

+// REQUIRES: msp430-registered-target
+// RUN: %clang_cc1 -mdisable-fp-elim -triple msp430 -S %s -o - | FileCheck %s --check-prefix=FP_ENFORCED
+// RUN: %clang_cc1 -triple msp430 -S %s -o - | FileCheck %s --check-prefix=FP_DEFAULT
+
+// Check the frame pointer is not used on MSP430 by default, but can be forcibly turned on.
+
+// FP_ENFORCED: push r4
+// FP_ENFORCED: mov r4, r4
+// FP_ENFORCED: pop r4
+// FP_DEFAULT: .globl fp_elim_check
+// FP_DEFAULT-NOT: push r4
+// FP_DEFAULT: mov r4, r4
+// FP_DEFAULT-NOT: pop r4
+
+void fp_elim_check()
+{
+  asm volatile ("mov r4, r4");
+}
+