[cfe-dev] [llvm-dev] [RFC] Allocatable Global Register Variables for ARM

[Amilendra Kodithuwakku via cfe-dev]

Thank you for your reply Eli,

I too was working with Carey on this feature, so please let me reply.

On 12/21/18 8:05 PM, Friedman, Eli via llvm-dev wrote:
> As a side-note, you might want to check that prologue/epilogue emission won't emit a PUSH/POP that refers to a register reserved this way; we sometimes add an "extra" register to align the stack.

Yes, you are right.
Checking determineCalleeSaves(), we see that it maintains a number of free Register pools, UnspilledCS1GPRs, UnspilledCS2GPRs and AvailableRegs, which are used to find Registers that can be used for the extra callee saves you mentioned.
There probably are more like this. Thanks for pointing that out.
We will investigate and extend our design.

>
> Why did you decide to use global metadata here?  For AArch64, we use a target feature instead, to implement roughly equivalent functionality (the reserve-x18 feature, to implement -ffixed-x18).
> Making a global register declaration have side-effects never made sense, IMO; on the surface, it's using variable declaration syntax, but in reality it's actually changing the ABI rules for the whole file.
> I would prefer to support -ffixed-r4, and never allow global register declarations to modify the ABI. This subset should be compatible with gcc, as far as I know.
>
> (Compiler flags that affect the ABI are easy to misuse, but clang and gcc have a long tradition of flags which change the ABI, so it's not really worse than what we already do.)
>
>
We were looking for a solution which works with LTO.
While we investigated a possible -ffixed-reg flag, our understanding was that it would only work as long as it sets module metadata in the IR.
Adding a -ffixed-reg option in the LLVM backend, and adding that option to the -cc1 command-line, would not work because that would not get passed through to the backend when LTO is used. So our belief was that one could later implement the -ffixed-reg flag upon the module metadata added by this patch.

Is this the target feature mechanism you explained? https://llvm.org/docs/WritingAnLLVMBackend.html#subtarget-support
I still have not gone through the specifics of that but do you know if it would work with LTO?

Thanks
Amilendra

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Friedman, Eli via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Friday, December 21, 2018 8:05 PM
To: Carey Williams; llvm-dev at lists.llvm.org
Cc: cfe-dev at lists.llvm.org
Subject: Re: [llvm-dev] [RFC] Allocatable Global Register Variables for ARM

On 12/21/2018 8:08 AM, Carey Williams via llvm-dev wrote:
Hi all,

This is a RFC on support for Global Register Variables in the Arm backend.

Whilst there has been some prior discussion about whether or not LLVM should (or even needs to) support global register variables,
today there seems to be a good measure of support for this in both Clang+LLVM (although it is currently limited to SP).
When most of this support landed there was some concern expressed around the difficulty of extending it to cover allocatable registers.
We have been looking at building atop of this current support to provide that ability to reserve allocatable registers.

Our primary (bare-metal) use-cases for such support are:

    1. Holding pointers for frequently-accessed data.
    2. Placement of secure values (e.g. checksums) in specific registers to prevent them from being written out to main memory.


As a side-note, you might want to check that prologue/epilogue emission won't emit a PUSH/POP that refers to a register reserved this way; we sometimes add an "extra" register to align the stack.


This proposal aims to provide support for using r4-r11 as global register variables. This involves adding them to the reserved register set
and preventing them from being callee-saved. We have deliberately tried to avoid registers that have a distinct ABI/AAPCS use,
such as call-clobbered registers, LR and PC etc. Naturally the current support for the stack-pointer remains.


Restricting this specifically to r4-r11 definitely makes sense; allowing other registers would be hard.


r7, r11 and r9 at least are special cases, and are mentioned in more detail below.

Clang Changes
----------------------
The main proposed functional change to Clang is the tracking of global register variable declarations via module flags.
Each declaration in a translation unit such as "register unsigned int foo __asm("r4");", will be mapped by the front-end to a module flag.

e.g.
---
!{i32 1, !"fixed_reg.foo", !"r4"}
---

This is achieved via a modification to CodeGenModule::EmitGlobal. In addition, there are some changes relating
to specifying valid global registers (by adding an Arm override for isValidGlobalRegister),

Draft Patch: https://reviews.llvm.org/D56003


Why did you decide to use global metadata here?  For AArch64, we use a target feature instead, to implement roughly equivalent functionality (the reserve-x18 feature, to implement -ffixed-x18).


Making a global register declaration have side-effects never made sense, IMO; on the surface, it's using variable declaration syntax, but in reality it's actually changing the ABI rules for the whole file.  I would prefer to support -ffixed-r4, and never allow global register declarations to modify the ABI. This subset should be compatible with gcc, as far as I know.


(Compiler flags that affect the ABI are easy to misuse, but clang and gcc have a long tradition of flags which change the ABI, so it's not really worse than what we already do.)


A Note on r7, r11 and r9
----------------------------------
Whilst generally considered allocatable, these registers can on occasion be reserved for other purposes.
As frame pointers, in the case of r7, and r11, and via -ffixed-r9 (or -frwpi), in the case of r9.

The attached patches do not currently provide any mitigations against these cases.
Options could range from disallowing these registers entirely, to throwing warnings or
trying to catch and error in the correct scenarios (e.g. usage -ffixed-r9 when r9 is declared as a global reg variable).


We probably want to emit an error here, to avoid confusion.


-Eli


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