[RISCV] Allow fp as an alias of s0

The RISC-V Assembly Programmer's Manual defines fp as another alias of x8.
However, our tablegen rules only recognise s0. This patch adds fp as another
alias of x8. GCC also accepts fp.

Differential Revision: https://reviews.llvm.org/D59209
Patch by Ferran Pallarès Roca.

llvm-svn: 355867

llvm/lib/Target/RISCV/RISCVRegisterInfo.td

@@ -55,7 +55,7 @@ let RegAltNameIndices = [ABIRegAltName] in {
   def X6  : RISCVReg<6, "x6", ["t1"]>, DwarfRegNum<[6]>;
   def X7  : RISCVReg<7, "x7", ["t2"]>, DwarfRegNum<[7]>;
   }
-  def X8  : RISCVReg<8, "x8", ["s0"]>, DwarfRegNum<[8]>;
+  def X8  : RISCVReg<8, "x8", ["s0", "fp"]>, DwarfRegNum<[8]>;
   def X9  : RISCVReg<9, "x9", ["s1"]>, DwarfRegNum<[9]>;
   def X10 : RISCVReg<10,"x10", ["a0"]>, DwarfRegNum<[10]>;
   def X11 : RISCVReg<11,"x11", ["a1"]>, DwarfRegNum<[11]>;

llvm/test/MC/RISCV/rvi-alternate-abi-names.s

@@ -0,0 +1,16 @@
+# RUN: llvm-mc %s -triple=riscv32 \
+# RUN:     | FileCheck %s
+# RUN: llvm-mc %s -triple=riscv64 \
+# RUN:     | FileCheck %s
+# RUN: llvm-mc -filetype=obj -triple riscv32 < %s \
+# RUN:     | llvm-objdump -d -r - \
+# RUN:     | FileCheck %s
+# RUN: llvm-mc -filetype=obj -triple riscv64 < %s \
+# RUN:     | llvm-objdump -d -r - \
+# RUN:     | FileCheck %s
+
+# 'fp' is an alternate ABI name for 's0' and it should be accepted in input.
+# However, 's0' should be printed in preference.
+
+# CHECK: addi s0, s0, -4
+addi fp, fp, -4