RISC-V: Avoid emitting invalid instructions in mixed RVC/no-RVC code

When linking the following code

    .global _prog_start
    _prog_start:
            mv x1, x1
            mv x2, x2
    .align 2
    rvc_boundry:
    .option norvc
    .align 3
            mv x3, x3

we currently emit an invalid two-byte 0 instruction.  The actual output
code looks like

    0000000080000000 <_prog_start>:
        80000000:   8086                    mv      ra,ra
        80000002:   810a                    mv      sp,sp

    0000000080000004 <rvc_boundry>:
        80000004:   0000                    unimp
        80000006:   0001                    nop
        80000008:   00018193                mv      gp,gp

This ends up manifesting due to the two-byte compressed NOP that's
pessimisticly emitted by the ".align 2", which results in "rvc_boundry"
being 2-byte aligned.  frag_align_code() then goes and outputs a 2-byte
NOP (which is invalid in no-RVC mode) to align the code back to a 4-byte
boundry, which can't be relaxed away by the linker as it's not part of
the R_RISCV_RELAX relocation.

The fix is to just always emit the worst case possible alignment into
the output as a single R_RISCV_RELAX, which the linker will then fix up.

With this patch I get the expected code generation

    0000000080000000 <_prog_start>:
        80000000:   8086                    mv      ra,ra
        80000002:   810a                    mv      sp,sp

    0000000080000004 <rvc_boundry>:
        80000004:   00000013                nop
        80000008:   00018193                mv      gp,gp

gas/ChangeLog

2017-09-07  Palmer Dabbelt  <palmer@dabbelt.com>

        * config/tc-riscv.c (riscv_frag_align_code): Emit the entire
        alignment sequence inside R_RISCV_ALIGN.

1 files changed, 8 insertions, 17 deletions

diff --git a/gas/config/tc-riscv.c b/gas/config/tc-riscv.c
index 4c644ab..1a15efc 100644
--- a/gas/config/tc-riscv.c
+++ b/gas/config/tc-riscv.c
@@ -2274,30 +2274,21 @@ bfd_boolean
 riscv_frag_align_code (int n)
 {
   bfd_vma bytes = (bfd_vma) 1 << n;
-  bfd_vma min_text_alignment_order = riscv_opts.rvc ? 1 : 2;
-  bfd_vma min_text_alignment = (bfd_vma) 1 << min_text_alignment_order;
-
-  /* First, get back to minimal alignment.  */
-  frag_align_code (min_text_alignment_order, 0);
+  bfd_vma worst_case_bytes = bytes - 2;
+  char *nops = frag_more (worst_case_bytes);
+  expressionS ex;
 
   /* When not relaxing, riscv_handle_align handles code alignment.  */
   if (!riscv_opts.relax)
     return FALSE;
 
-  if (bytes > min_text_alignment)
-    {
-      bfd_vma worst_case_bytes = bytes - min_text_alignment;
-      char *nops = frag_more (worst_case_bytes);
-      expressionS ex;
+  ex.X_op = O_constant;
+  ex.X_add_number = worst_case_bytes;
 
-      ex.X_op = O_constant;
-      ex.X_add_number = worst_case_bytes;
+  riscv_make_nops (nops, worst_case_bytes);
 
-      riscv_make_nops (nops, worst_case_bytes);
-
-      fix_new_exp (frag_now, nops - frag_now->fr_literal, 0,
-		   &ex, FALSE, BFD_RELOC_RISCV_ALIGN);
-    }
+  fix_new_exp (frag_now, nops - frag_now->fr_literal, 0,
+	       &ex, FALSE, BFD_RELOC_RISCV_ALIGN);
 
   return TRUE;
 }