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<div>Hi all,<br>
<br>
This is a RFC on support for Global Register Variables in the Arm backend.<br>
<br>
Whilst there has been some prior discussion about whether or not LLVM should (or even needs to) support global register variables,<br>
today there seems to be a good measure of support for this in both Clang+LLVM (although it is currently limited to SP).<br>
When most of this support landed there was some concern expressed around the difficulty of extending it to cover allocatable registers.<br>
We have been looking at building atop of this current support to provide that ability to reserve allocatable registers.<br>
<br>
Our primary (bare-metal) use-cases for such support are:<br>
<br>
1. Holding pointers for frequently-accessed data.<br>
2. Placement of secure values (e.g. checksums) in specific registers to prevent them from being written out to main memory.<br>
<br>
This proposal aims to provide support for using r4-r11 as global register variables. This involves adding them to the reserved register set<br>
and preventing them from being callee-saved. We have deliberately tried to avoid registers that have a distinct ABI/AAPCS use,<br>
such as call-clobbered registers, LR and PC etc. Naturally the current support for the stack-pointer remains.<br>
<br>
r7, r11 and r9 at least are special cases, and are mentioned in more detail below.<br>
<br>
Clang Changes<br>
<span>----------------------</span><br>
The main proposed functional change to Clang is the tracking of global register variable declarations via module flags.<br>
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.<br>
<br>
e.g.<br>
---<br>
!{i32 1, !"fixed_reg.foo", !"r4"}<br>
---<br>
<br>
This is achieved via a modification to CodeGenModule::EmitGlobal. In addition, there are some changes relating<br>
to specifying valid global registers (by adding an Arm override for isValidGlobalRegister),<br>
<br>
Draft Patch: <a href="https://reviews.llvm.org/D56003" target="_blank" rel="noopener noreferrer" class="x_OWAAutoLink">
https://reviews.llvm.org/D56003</a><br>
<br>
LLVM Changes<br>
----------------------<br>
In the LLVM backend we can then use the module flags to add any (valid) specified global registers to the<br>
reserved reg list (getReservedRegs). In addition, to satisfy the second use-case*, we use the flags to remove<br>
any (valid) global registers from the list of callee registers to be saved (determineCalleeSaves).<br>
This shouldn't have any negative impact on a register that is already reserved for use.<br>
<br>
GCC seems to exhibit similar behaviour: <br>
<br>
"If the register is a call-saved register, call ABI is affected: the register will not be restored in function epilogue sequences after the variable has been assigned."<br>
See: https://gcc.gnu.org/onlinedocs/gcc/Global-Register-Variables.html#Global-Register-Variables<br>
<br>
Draft Patch: <a href="https://reviews.llvm.org/D56005" target="_blank" rel="noopener noreferrer" class="x_OWAAutoLink">
https://reviews.llvm.org/D56005</a><br>
<br>
A Note on r7, r11 and r9<br>
<span>----------------------</span>------------<br>
Whilst generally considered allocatable, these registers can on occasion be reserved for other purposes.<br>
As frame pointers, in the case of r7, and r11, and via -ffixed-r9 (or -frwpi), in the case of r9.<br>
<br>
The attached patches do not currently provide any mitigations against these cases.<br>
Options could range from disallowing these registers entirely, to throwing warnings or<br>
trying to catch and error in the correct scenarios (e.g. usage -ffixed-r9 when r9 is declared as a global reg variable).<br>
<br>
GCC's behaviour for many registers (at least notably, the call-clobbered registers, e.g. r0-r3), is to throw a warning, rather than an error.<br>
<br>
Other Notes<br>
<span>----------------------</span><br>
- It's worth noting that we may also look into extending this to cover AArch64 as well, in the near future.<br>
- Extending this proposal to work with a -ffixed-reg option should be feasible (if desired).<br>
- GCC warns when two global variables refer to the same register - this implementation silently accepts it.<br>
<br>
Previous Patches/Discussion<br>
-------------------------------<span>-----------</span><br>
- https://reviews.llvm.org/D3261<br>
- https://reviews.llvm.org/D3797<br>
- http://lists.llvm.org/pipermail/llvm-dev/2014-March/071472.html</div>
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