https://sourceware.org/bugzilla/show_bug.cgi?id=32705
Bug ID: 32705
Summary: RISC-V: objdump prints .insn due to incorrect $x
mapping symbol handling
Product: binutils
Version: unspecified
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: binutils
Assignee: unassigned at sourceware dot org
Reporter: andrewoates at gmail dot com
Target Milestone: ---
Symptom: objdump will fail to decode instructions when disassembling a file
generated by LLVM that mixes architecture strings between sections. This can
result in many ".insn" lines.
Root cause: LLVM emits "$x" mapping symbols to signal transitions from a custom
architecture string (such as one that does not include the "c" extension) back
to the default. When objdump sees a bare "$x" (as opposed to a "$xrv..." that
includes the architecture string), it does not actually reset to the ELF file
default. This causes subsequent instructions to be decoded based on whatever
the last-seen architecture string was.
AFAICT, gas will never emit object files like this, because it always emits a
full mapping string ("$xrv...") at the start of each section (see
gas/config/tc-riscv.c:624 [1]). LLVM's assembler will, however.
Alternatively, it's possible this is a strange interaction between LLVM's
output and ld when linking gas-generated and LLVM-generated object files
together --- but I think this is really an objdump bug.
I have a patch prepared to fix this, but filing the bug for reference. I'm
still working on a test case (which has to have a binary object file,
unfortunately, as gas won't generate a symbol sequence that triggers this bug).
Simple test case:
$ cat asm.s
.attribute arch, "rv64i"
.section .text, "ax"
addi a0, zero, 1 # $xrv64i
$ cat test.c
int func(int x) { return x + 1; }
$ ../gas/as-new -o /tmp/test.o asm.s
$ clang -c -O1 -target riscv64 -o /tmp/test2.o test.c
$ ../ld/ld-new /tmp/test.o /tmp/test2.o -o /tmp/linked.o
../ld/ld-new: warning: cannot find entry symbol _start; defaulting to
00000000000100e8
$ ../binutils/objdump -d /tmp/linked.o| less
fix-riscv-objdump () U
/tmp/linked.o: file format elf64-littleriscv
Disassembly of section .text:
00000000000100e8 <func-0x4>:
100e8: 00100513 li a0,1
00000000000100ec <func>:
100ec: 2505 .insn 2, 0x2505
100ee: 8082 .insn 2, 0x8082
If you alter asm.s to say "rv64ic", then you get the expected disassembly at
the end:
00000000000100ec <func>:
100ec: 2505 addiw a0,a0,1
100ee: 8082 ret
[1]
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gas/config/tc-riscv.c;h=493c393f5b2730fe8f841da074fa106d655ec9a7;hb=HEAD#l624
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