[llvm-dev] RFC: Dynamically Allocated "Callee Saved Registers" Lists

Mon Jan 9 10:42:30 PST 2017

Thanks Bruce for your comments.

Unfortunately there are calling conventions that require a callee to preserve a register that was used for argument passing.
For example: hhvm (passes and preserves R12), swift (passes and preserves R12 & R13).

Apparently there are other calling conventions that can use the proposed solution.
For example: Interrupt handler CC that needs to preserve all registers (instead of passed/returned arguments).

Regarding:
fnCalleeSavedRegs = archCSR & ~fnArgs
fnScratchRegs = ~fnCalleeSavedRegs & ~archSpecialRegs; // if not included in CSR

Your solution is the approach I started with while updating around 50 places were “fnCalleeSavedRegs” is calculated, but I encountered several issues:

1.      Creating “fnArgs” every time is expensive because it requires calling the CC functions every time.

2.      So “fnArgs” is bitmap that needs to be created and stored somewhere.

3.      Not in every place that we use “archCSR” we have (or can calculate) fnArgs because in some cases we don’t have the Machine Function handler.

Basically, my proposed solution is: Instead of calculating “fnArgs” and “fnCalleeSavesRegs” every time – Calculate “fnCalleeSavesRegs” once and save it.

Thanks again,
Oren

From: bruce.hoult at gmail.com [mailto:bruce.hoult at gmail.com] On Behalf Of Bruce Hoult
Sent: Monday, January 09, 2017 11:47
To: Ben Simhon, Oren <oren.ben.simhon at intel.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] RFC: Dynamically Allocated "Callee Saved Registers" Lists

Weird calling convention, but I see that's as documented at e.g.

https://software.intel.com/en-us/node/522787

Silly question:

Is there any calling convention, on any supported platform, that requires the callee to preserve a register that was used to pass an argument?

Of course someone *could* define such a CC, but if there isn't currently one, then we don't need to support that possibility *yet*.

If no one has done that then no need to add a whole new special case just for IA32 __regcall. We could just for every function do something like (assuming a bitmap representation):

fnCalleeSavedRegs = archCSR & ~fnArgs
fnScratchRegs = ~fnCalleeSavedRegs & ~archSpecialRegs; // if not included in CSR

This is simple enough to be calculated whenever it's needed, not stored.

On Mon, Jan 9, 2017 at 12:09 PM, Ben Simhon, Oren via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Dynamically Allocated “Callee Saved Registers” Lists

Each Calling convention (CC) defines a static list of registers that should be preserved by a callee function. All other registers should be saved by the caller.
Some CCs use additional condition: If the register is used for passing/returning arguments – the caller needs to save it - even if it is part of the Callee Saved Registers (CSR) list.
For example consider the following function:
void __regcall func(int a, int b, int c, int d, int e);
According to RegCall CC, parameters d and e should reside in registers EDI and ESI. The problem is that these registers also appear in the CSR list of RegCall calling convention. So, since the registers were used to pass arguments the callee doesn’t have to preserve their values.
The current LLVM implementation doesn’t support it. It will save a register if it is part of the static CSR list and will not care if the register is passed/returned by the callee.

There are two types of static CSR lists:

1.      register mask array of the CSRs (including register aliases)

2.      register CSR list

The proposed solution is to dynamically allocate the CSR lists (Only for these CCs). The lists will be updated with actual registers that should be saved by the callee.

Since we need the allocated lists to live as long as the function exists, the list should reside inside the Machine Register Info (MRI) which is a property of the Machine Function and managed by it (and has the same life span).

The lists should be saved in the MRI and populated upon LowerCall and LowerFormalArguments.

Open Issue

Machine Instructions (MI) have intermediate representation that can be printed and later on parsed to recreate the MIs.
MI printer and parser expect the Register Mask array pointer to point to a predefined (static) list of RegMasks. Those lists are retrieved from auto generated file x86GenRegisterInfo.inc using the functions: getRegMasks() and getRegMaskNames().
However, since we create a dynamically allocated register mask, its pointer will not reside in the static lists and no corresponding name could be found.
In that case, the MIPrinter will fail to emit the RegMask Name.

I would appreciate the community opinion regarding my solution and regarding possible solutions to the open issue.

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