[RISCV] Add obsolete aliases of fscsr, frcsr (fssr, frsr)

[edward-jones]

Summary

These old aliases were renamed, but are still used by some projects (eg newlib).

[edward-jones]

A comment on github by Andrew Waterman suggests that these aliases are expected to be supported by the tools even if they are obsolete. At the very least newlib still seems to use them.

[asb]

Thanks Ed, could you add some tests to test/MC/RISCV/rvf-aliases-valid.s please?

[edward-jones]

Added tests!

[asb]

LGTM, thanks!

llvm/lib/Target/RISCV/RISCVInstrInfoF.td

 
 // The following csr instructions actually alias instructions from the base ISA.
 // However, it only makes sense to support them when the F extension is enabled.
 // NOTE: "frcsr", "frrm", and "frflags" are more specialized version of "csrr".
 def : InstAlias<"frcsr $rd",      (CSRRS GPR:$rd, FCSR.Encoding, X0), 2>;
 def : InstAlias<"fscsr $rd, $rs", (CSRRW GPR:$rd, FCSR.Encoding, GPR:$rs)>;
 def : InstAlias<"fscsr $rs",      (CSRRW      X0, FCSR.Encoding, GPR:$rs), 2>;
 
+// frsr, fssr are obsolete aliases replaced by frcsr, fscsr, so give them
+// zero weight.
+def : InstAlias<"frsr $rd",       (CSRRS GPR:$rd, FCSR.Encoding, X0), 0>;
+def : InstAlias<"fssr $rd, $rs",  (CSRRW GPR:$rd, FCSR.Encoding, GPR:$rs), 0>;
+def : InstAlias<"fssr $rs",       (CSRRW      X0, FCSR.Encoding, GPR:$rs), 0>;
+
 def : InstAlias<"frrm $rd",        (CSRRS  GPR:$rd, FRM.Encoding, X0), 2>;
 def : InstAlias<"fsrm $rd, $rs",   (CSRRW  GPR:$rd, FRM.Encoding, GPR:$rs)>;
 def : InstAlias<"fsrm $rs",        (CSRRW       X0, FRM.Encoding, GPR:$rs), 2>;
 def : InstAlias<"fsrmi $rd, $imm", (CSRRWI GPR:$rd, FRM.Encoding, uimm5:$imm)>;
 def : InstAlias<"fsrmi $imm",      (CSRRWI      X0, FRM.Encoding, uimm5:$imm), 2>;
 
 def : InstAlias<"frflags $rd",        (CSRRS  GPR:$rd, FFLAGS.Encoding, X0), 2>;
 def : InstAlias<"fsflags $rd, $rs",   (CSRRW  GPR:$rd, FFLAGS.Encoding, GPR:$rs)>;

llvm/test/MC/RISCV/rvf-aliases-valid.s

 frcsr x5
 # CHECK-INST: csrrw t1, fcsr, t2
 # CHECK-ALIAS: fscsr t1, t2
 fscsr x6, x7
 # CHECK-INST: csrrw  zero, fcsr, t3
 # CHECK-ALIAS: fscsr t3
 fscsr x28
 
+# These are obsolete aliases of frcsr/fscsr. They are accepted by the assembler
+# but the disassembler should always print them as the equivalent, new aliases.
+# CHECK-INST: csrrs t4, fcsr, zero
+# CHECK-ALIAS: frcsr t4
+frsr x29
+# CHECK-INST: csrrw t5, fcsr, t6
+# CHECK-ALIAS: fscsr t5, t6
+fssr x30, x31
+# CHECK-INST: csrrw zero, fcsr, s0
+# CHECK-ALIAS: fscsr s0
+fssr x8
+
 # CHECK-INST: csrrs t4, frm, zero
 # CHECK-ALIAS: frrm t4
 frrm x29
 # CHECK-INST: csrrw  t5, frm, t4
 # CHECK-ALIAS: fsrm t5, t4
 fsrm x30, x29
 # CHECK-INST: csrrw  zero, frm, t6
 # CHECK-ALIAS: fsrm t6