[MSP430] Declare comparison LibCalls as returning i16 instead of i32

[atrosinenko]

Summary

For TI's distribution of msp430-gcc

msp430-elf-gcc -S -o- -Os -x c - <<< "int f(float a, float b) { return a != b; }"

does not mention R13 at all. __libgcc_cmp_return__ machine mode is 2 byte on MSP430, as well.

[atrosinenko]

Just a note: using __mspabi_cmpf as a generic LibCall seems quite incorrect because __mspabi_cmpf does not accept NaNs according to MSP430 EABI. msp430-elf-gcc v9.2.0 just uses __nesf2 in the example above. I plan changing this later when doing more adjustments for libgcc/clang_rt.

llvm/lib/Target/MSP430/MSP430ISelLowering.h

 //===-- MSP430ISelLowering.h - MSP430 DAG Lowering Interface ----*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
namespace llvm {
   public:
     explicit MSP430TargetLowering(const TargetMachine &TM,
                                   const MSP430Subtarget &STI);
 
     MVT getScalarShiftAmountTy(const DataLayout &, EVT) const override {
       return MVT::i8;
     }
 
+    MVT::SimpleValueType getCmpLibcallReturnType() const override {
+      return MVT::i16;
+    }
+
     /// LowerOperation - Provide custom lowering hooks for some operations.
     SDValue LowerOperation(SDValue Op, SelectionDAG &DAG) const override;
 
     /// getTargetNodeName - This method returns the name of a target specific
     /// DAG node.
     const char *getTargetNodeName(unsigned Opcode) const override;
 
     SDValue LowerShifts(SDValue Op, SelectionDAG &DAG) const;

llvm/test/CodeGen/MSP430/cmp-return-type.ll

+; RUN: llc < %s | FileCheck %s
+
+target datalayout = "e-m:e-p:16:16-i32:16-i64:16-f32:16-f64:16-a:8-n8:16-S16"
+target triple = "msp430"
+
+define i16 @f(float %a, float %b) optsize {
+  %cmp = fcmp une float %a, %b
+  %conv = zext i1 %cmp to i16
+; CHECK-LABEL: call #__mspabi_cmpf
+; CHECK-NOT:   r13
+; CHECK-LABEL: mov r2
+
+; This is quite fragile attempt to detect the return type:
+; Correct:
+;        call    #__mspabi_cmpf
+;        tst     r12
+;        mov     r2, r13
+; Incorrect:
+;        call    #__mspabi_cmpf
+;        bis     r12, r13        <-- checking (R12:R13)
+;        tst     r13
+;        mov     r2, r13
+
+  ret i16 %conv
+}