Allow fp as an alias of s0

[fpallares]

Summary

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.

[asb]

Thanks, I'd missed the fp alias as I was going by the riscv-elf-psabi-doc primarily.

I think rvi-alternate-abi-names.s would be a better name for the test. You can then add a comment explaining that: fp is an alternate ABI name for s0 it should be accepted in input, but s0 should be printed in preference. This is of course the behaviour your patch demonstrates!

[fpallares]

Thank you Alex. I've updated the test name and I've added a comment.
Hope it looks better now.

[asb]

Looks good to me, thanks!

[fpallares]

Great, thank you for reviewing!

lib/Target/RISCV/RISCVRegisterInfo.td

let RegAltNameIndices = [ABIRegAltName] in {
   def X1  : RISCVReg<1, "x1", ["ra"]>, DwarfRegNum<[1]>;
   def X2  : RISCVReg<2, "x2", ["sp"]>, DwarfRegNum<[2]>;
   def X3  : RISCVReg<3, "x3", ["gp"]>, DwarfRegNum<[3]>;
   def X4  : RISCVReg<4, "x4", ["tp"]>, DwarfRegNum<[4]>;
   def X5  : RISCVReg<5, "x5", ["t0"]>, DwarfRegNum<[5]>;
   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]>;
   def X12 : RISCVReg<12,"x12", ["a2"]>, DwarfRegNum<[12]>;
   def X13 : RISCVReg<13,"x13", ["a3"]>, DwarfRegNum<[13]>;
   def X14 : RISCVReg<14,"x14", ["a4"]>, DwarfRegNum<[14]>;
   def X15 : RISCVReg<15,"x15", ["a5"]>, DwarfRegNum<[15]>;
   let CostPerUse = 1 in {

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

+# 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