LLVM Toolchain not generating mapping symbols

[ziuziakowska]

This should be fixed when we update our LLVM toolchain.

Currently, files assembled or compiled by the LLVM toolchain do not contain mapping symbols for regions of code that use a different set of extensions - of interest here is the unratified bit-manipulation extensions - controlled with .option arch directives.

This causes llvm-objdump and Binutils objdump to be unable to disassemble the instructions in those extensions unless given the appropriate --mattr flags, as the regions are not marked as using them. as in Binutils 2.44 generates these symbols and is able to disassemble the instructions just fine.

Demonstration:

Using a test file, test_syms.S:

zbp:
	.option push
	.option arch, +zbp0p93
	grev a0, a1, a2
	.option pop

Assembled with Binutils as and Clang, respectively:
$ ./lowrisc-toolchains/build/dist/bin/riscv32-unknown-elf-as -march=rv32i test_syms.S -o test_syms_binutils.o
$ bazel run @lowrisc_rv32imcb_toolchain//:bin/clang -- -march=rv32i -c ~/test_syms.S -o ~/test_syms_llvm.o

Binutils objdump is able to disassemble the instruction when assembled with as but not Clang:

$ ./lowrisc-toolchains/build/dist/bin/riscv32-unknown-elf-objdump -d test_syms_binutils.o

test_syms_binutils.o:     file format elf32-littleriscv

Disassembly of section .text:

00000000 <zbp>:
   0:	68c5d533          	grev	a0,a1,a2

$ ./lowrisc-toolchains/build/dist/bin/riscv32-unknown-elf-objdump -d test_syms_llvm.o

test_syms_llvm.o:     file format elf32-littleriscv

Disassembly of section .text:

00000000 <zbp>:
   0:	68c5d533          	.insn	4, 0x68c5d533

llvm-objdump is not able to disassemble the instruction in either file, unless instructed to with --mattr=+experimental-zbp:

$ bazel run @lowrisc_rv32imcb_toolchain//:bin/llvm-objdump -- -d ~/test_syms_binutils.o
$ bazel run @lowrisc_rv32imcb_toolchain//:bin/llvm-objdump -- -d ~/test_syms_llvm.o

Disassembly of section .text:

00000000 <zbp>:
       0: 33 d5 c5 68  	<unknown>

$ bazel run @lowrisc_rv32imcb_toolchain//:bin/llvm-objdump -- --mattr=+experimental_zbp -d ~/test_syms_binutils.o
$ bazel run @lowrisc_rv32imcb_toolchain//:bin/llvm-objdump -- --mattr=+experimental_zbp -d ~/test_syms_llvm.o

~/test_syms_llvm.o:	file format elf32-littleriscv

Disassembly of section .text:

00000000 <zbp>:
       0: 33 d5 c5 68  	grev	a0, a1, a2

Both toolchain disassemblers show that the file assembled by the LLVM toolchain is missing the appropriate mapping symbol:

$ ./lowrisc-toolchains/build/dist/bin/riscv32-unknown-elf-objdump --syms --special-syms test_syms_binutils.o
$ bazel run @lowrisc_rv32imcb_toolchain//:bin/llvm-objdump -- --syms ~/test_syms_binutils.o

~/test_syms_binutils.o:	file format elf32-littleriscv

SYMBOL TABLE:
00000000 l    d  .text	00000000 .text
00000000 l    d  .data	00000000 .data
00000000 l    d  .bss	00000000 .bss
00000000 l       .text	00000000 zbp
00000000 l       .text	00000000 $xrv32i2p1_zbp0p93
00000000 l    d  .riscv.attributes	00000000 .riscv.attributes

$ ./lowrisc-toolchains/build/dist/bin/riscv32-unknown-elf-objdump --syms --special-syms test_syms_llvm.o
$ bazel run @lowrisc_rv32imcb_toolchain//:bin/llvm-objdump -- --syms ~/test_syms_llvm.o

~/test_syms_llvm.o:	file format elf32-littleriscv

SYMBOL TABLE:
00000000 l       .text	00000000 zbp