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[PATCH 2/2] RISC-V: Avoid emitting invalid instructions in mixed RVC/no-RVC code
- From: Palmer Dabbelt <palmer at dabbelt dot com>
- To: binutils at sourceware dot org
- Cc: patches at groups dot riscv dot org, Palmer Dabbelt <palmer at dabbelt dot com>
- Date: Wed, 6 Sep 2017 13:43:48 -0700
- Subject: [PATCH 2/2] RISC-V: Avoid emitting invalid instructions in mixed RVC/no-RVC code
- Authentication-results: sourceware.org; auth=none
- References: <20170906204348.15138-1-palmer@dabbelt.com>
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
2017-09-06 Palmer Dabbelt <palmer@dabbelt.com>
* config/tc-riscv.c (riscv_frag_align_code): Emit the entire
alignment sequence inside R_RISCV_ALIGN.
---
gas/config/tc-riscv.c | 25 ++++++++-----------------
1 file changed, 8 insertions(+), 17 deletions(-)
diff --git a/gas/config/tc-riscv.c b/gas/config/tc-riscv.c
index 4c644ab65edf..1a15efc47985 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;
}
--
2.13.5