Dear Jiong,
that's a great work.
I havent' gone through the whole patches, but it seems to me that the
documentation is not that much.
From my past experiences, putting your hands into a compiler without at
least some high-level documentation that presents how it works, it would
be a nightmare.
Even something like what you wrote in this email is valuable; of course,
also how to turn this feature on.
Thanks,
fulvio
On 18/09/2017 13:47, Jiong Wang via iovisor-dev wrote:
Hi,
Currently, LLVM eBPF backend always generate code in 64-bit mode, this may
cause troubles when JITing to 32-bit targets.
For example, it is quite common for XDP eBPF program to access some packet
fields through base + offset that the default eBPF will generate BPF_ALU64 for
the address formation, later when JITing to 32-bit hardware, BPF_ALU64 needs
to be expanded into 32 bit ALU sequences even though the address space is
32-bit that the high bits is not significant.
While a complete 32-bit mode implemention may need an new ABI (something like
-target-abi=ilp32), this patch set first add some initial code so we could
construct 32-bit eBPF tests through hand-written assembly.
A new 32-bit register set is introduced, its name is with "w" prefix and LLVM
assembler will encode statements like "w1 += w2" into the following 8-bit code
field:
BPF_ADD | BPF_X | BPF_ALU
BPF_ALU will be used instead of BPF_ALU64.
NOTE, currently you can only use "w" register with ALU statements, not with
others like branches etc as they don't have different encoding for 32-bit
target.
Comments?
*** BLURB HERE ***
Jiong Wang (4):
Improve instruction encoding descriptions
Improve class inheritance in instruction patterns
New 32-bit register set
Initial 32-bit ALU (BPF_ALU) encoding support in assembler
lib/Target/BPF/BPFInstrFormats.td | 84 +++-
lib/Target/BPF/BPFInstrInfo.td | 506 +++-
lib/Target/BPF/BPFRegisterInfo.td | 74 +++-
lib/Target/BPF/Disassembler/BPFDisassembler.cpp | 15 +
test/MC/BPF/insn-unit-32.s | 53 +++
5 files changed, 427 insertions(+), 305 deletions(-)
create mode 100644 test/MC/BPF/insn-unit-32.s
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