Kai Wang via llvm-dev
2020-Oct-06 03:11 UTC
[llvm-dev] [MC] Questions about relaxation in MC
Hi all, In RISC-V ISA, the range of conditional branches is within 4KiB. In current implementation, if the branch target is out of range, LLVM MC will issue an error message to tell users it could not resolve the fixup record. I have compared the result with the GNU assembler. GNU assembler will convert the branch to inverted one plus jump to make the branch possible. The range of unconditional jump is 1MiB. It looks like ########################## bne a0, a1, FAR_BRANCH … FAR_BRANCH: converted to ########################## beq a0, a1, SKIP_J j FAR_BRANCH SKIP_J: … FAR_BRANCH: I found there is a target hook, relaxInstruction, that tries to achieve the similar goal. However, the target hook only replaces one MCInst with another one with a larger branch range. For example, c.beqz will be converted to beq in the RISC-V backend if the fixup value is out of range. There seems no target hook to convert one MCInst to a complex pattern in LLVM MC. Do I miss something obvious? I found there is a target hook, finishLayout, to manipulate the code generated. Does it make sense to implement the feature in finishLayout? Or is there any better idea to achieve the conversion? Thanks a lot. Best, Kai -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20201006/42aac2d1/attachment.html>
Correct the title. On Tue, Oct 6, 2020 at 11:11 AM Kai Wang <kai.wang at sifive.com> wrote:> Hi all, > > In RISC-V ISA, the range of conditional branches is within 4KiB. In > current implementation, if the branch target is out of range, LLVM MC will > issue an error message to tell users it could not resolve the fixup record. > I have compared the result with the GNU assembler. GNU assembler will > convert the branch to inverted one plus jump to make the branch possible. > The range of unconditional jump is 1MiB. It looks like > > ########################## > bne a0, a1, FAR_BRANCH > … > FAR_BRANCH: > > converted to > > ########################## > beq a0, a1, SKIP_J > j FAR_BRANCH > SKIP_J: > … > FAR_BRANCH: > > I found there is a target hook, relaxInstruction, that tries to achieve > the similar goal. However, the target hook only replaces one MCInst with > another one with a larger branch range. For example, c.beqz will be > converted to beq in the RISC-V backend if the fixup value is out of range. > There seems no target hook to convert one MCInst to a complex pattern in > LLVM MC. Do I miss something obvious? > > I found there is a target hook, finishLayout, to manipulate the code > generated. Does it make sense to implement the feature in finishLayout? Or > is there any better idea to achieve the conversion? Thanks a lot. > > Best, > Kai >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20201006/99c879a4/attachment.html>
This sounds very similar to what PowerPC does, see: https://github.com/llvm/llvm-project/blob/master/llvm/lib/Target/PowerPC/PPCBranchSelector.cpp However, PowerPC is doing this in the compiler backend, not in the assembler. One issue is that the process must be iterative, because branches can go both forward and backward, and replacing some branch instructions with the branch pairs can then cause other branches to go out of range. -Hal On 10/5/20 10:20 PM, Kai Wang via llvm-dev wrote:> Correct the title. > > On Tue, Oct 6, 2020 at 11:11 AM Kai Wang <kai.wang at sifive.com > <mailto:kai.wang at sifive.com>> wrote: > > Hi all, > > In RISC-V ISA, the range of conditional branches is within 4KiB. > In current implementation, if the branch target is out of range, > LLVM MC will issue an error message to tell users it could not > resolve the fixup record. I have compared the result with the GNU > assembler. GNU assembler will convert the branch to inverted one > plus jump to make the branch possible. The range of unconditional > jump is 1MiB. It looks like > > ########################## > bne a0, a1, FAR_BRANCH > … > FAR_BRANCH: > > converted to > > ########################## > beq a0, a1, SKIP_J > j FAR_BRANCH > SKIP_J: > … > FAR_BRANCH: > > I found there is a target hook, relaxInstruction, that tries to > achieve the similar goal. However, the target hook only replaces > one MCInst with another one with a larger branch range. For > example, c.beqz will be converted to beq in the RISC-V backend if > the fixup value is out of range. There seems no target hook to > convert one MCInst to a complex pattern in LLVM MC. Do I miss > something obvious? > > I found there is a target hook, finishLayout, to manipulate the > code generated. Does it make sense to implement the feature in > finishLayout? Or is there any better idea to achieve the > conversion? Thanks a lot. > > Best, > Kai > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev-- Hal Finkel Lead, Compiler Technology and Programming Languages Leadership Computing Facility Argonne National Laboratory -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20201006/51ad78cb/attachment.html>
Philip Reames via llvm-dev
2020-Oct-07 15:20 UTC
[llvm-dev] Questions about relaxation in MC
If done in the assembler, this is branch relaxation. You need to implement the calls backs mayNeedRelaxation, and relaxInstruction on your target MCAsmBackend. The iteration relaxation logic already exists, see the generic MC code. Philip On 10/5/20 8:20 PM, Kai Wang via llvm-dev wrote:> Correct the title. > > On Tue, Oct 6, 2020 at 11:11 AM Kai Wang <kai.wang at sifive.com > <mailto:kai.wang at sifive.com>> wrote: > > Hi all, > > In RISC-V ISA, the range of conditional branches is within 4KiB. > In current implementation, if the branch target is out of range, > LLVM MC will issue an error message to tell users it could not > resolve the fixup record. I have compared the result with the GNU > assembler. GNU assembler will convert the branch to inverted one > plus jump to make the branch possible. The range of unconditional > jump is 1MiB. It looks like > > ########################## > bne a0, a1, FAR_BRANCH > … > FAR_BRANCH: > > converted to > > ########################## > beq a0, a1, SKIP_J > j FAR_BRANCH > SKIP_J: > … > FAR_BRANCH: > > I found there is a target hook, relaxInstruction, that tries to > achieve the similar goal. However, the target hook only replaces > one MCInst with another one with a larger branch range. For > example, c.beqz will be converted to beq in the RISC-V backend if > the fixup value is out of range. There seems no target hook to > convert one MCInst to a complex pattern in LLVM MC. Do I miss > something obvious? > > I found there is a target hook, finishLayout, to manipulate the > code generated. Does it make sense to implement the feature in > finishLayout? Or is there any better idea to achieve the > conversion? Thanks a lot. > > Best, > Kai > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20201007/a7037474/attachment.html>