[llvm-dev] [ARM] Register pressure with -mthumb forces register reload before each call

Tue Apr 7 11:25:25 PDT 2020

If I'm understanding what's going on in this test correctly, what's happening is:
 * ARMTargetLowering::LowerCall prefers indirect calls when a function is called at least 3 times in minsize
 * In thumb 1 (without -fno-omit-frame-pointer) we have effectively only 3 callee-saved registers (r4-r6)
 * The function has three arguments, so those three plus the register we need to hold the function address is more than our callee-saved registers
 * Therefore something needs to be spilt
 * The function address can be rematerialized, so we spill that and insert and LDR before each call

If we didn't have this spilling happening (e.g. if the function had one less argument) then the code size of using BL vs BLX
 * BL: 3*4-byte BL = 12 bytes
 * BX: 3*2-byte BX + 1*2-byte LDR + 4-byte litpool = 12 bytes
(So maybe even not considering spilling, LowerCall should be adjusted to do this for functions called 4 or more times)

When we have to spill, if we compare spilling the functions address vs spilling an argument:
 * BX with spilt fn: 3*2-byte BX + 3*2-byte LDR + 4-byte litpool = 16 bytes
 * BX with spilt arg: 3*2-byte BX + 1*2-byte LDR + 4-byte litpool + 1*2-byte STR + 2*2-byte LDR = 18 bytes
So just changing the spilling heuristic won't work.

The two ways I see of fixing this:
 * In LowerCall only prefer an indirect call if the number of integer register arguments is less than the number of callee-saved registers.
 * When the load of the function address is spilled, instead of just rematerializing the load instead convert the BX back into BL.

The first of these would be easier, but there will be situations where we need to use less than three callee-saved registers (e.g. arguments are loaded from a pointer) and there are situations where we will spill the function address for reasons entirely unrelated to the function arguments (e.g. if we have enough live local variables).

For the second, looking at InlineSpiller.cpp it does have the concept of rematerializing by folding a memory operand into another instruction, so I think we could make use of that to do this. It looks like it would involve adding a foldMemoryOperand function to ARMInstrInfo and then have this fold a LDR into a BX by turning it into a BL.

John

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Prathamesh Kulkarni via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 07 April 2020 21:07
To: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [ARM] Register pressure with -mthumb forces register reload before each call

On Tue, 31 Mar 2020 at 22:03, Prathamesh Kulkarni
<prathamesh.kulkarni at linaro.org> wrote:
>
> Hi,
> Compiling attached test-case, which is reduced version of of
> uECC_shared_secret from tinycrypt library [1], with
> --target=arm-linux-gnueabi -march=armv6-m -Oz -S
> results in reloading of register holding function's address before
> every call to blx:
>
>         ldr       r3, .LCPI0_0
>         blx      r3
>         mov    r0, r6
>         mov    r1, r5
>         mov    r2, r4
>         ldr       r3, .LCPI0_0
>         blx       r3
>         ldr        r3, .LCPI0_0
>         mov     r0, r6
>         mov     r1, r5
>         mov     r2, r4
>         blx       r3
>
> .LCPI0_0:
>         .long   foo
>
> From dump of regalloc (attached), AFAIU, what seems to happen during
> greedy allocator is, all virt regs %0 to %3 are live across first two
> calls to foo. Thus %0, %1 and %2 get assigned r6, r5 and r4
> respectively, and %3 which holds foo's address doesn't have any
> register left.
> Since it's live-range has least weight, it does not evict any existing interval,
> and gets split. Eventually we have the following allocation:
>
> [%0 -> $r6] tGPR
> [%1 -> $r5] tGPR
> [%2 -> $r4] tGPR
> [%6 -> $r3] tGPR
> [%11 -> $r3] tGPR
> [%16 -> $r3] tGPR
> [%17 -> $r3] tGPR
>
> where %6, %11, %16 and %17 all are derived from %3.
> And since r3 is a call-clobbered register, the compiler is forced to
> reload foo's address
> each time before blx.
>
> To fix this, I thought of following approaches:
> (a) Disable the heuristic to prefer indirect call when there are at
> least 3 calls to
> same function in basic block in ARMTargetLowering::LowerCall for Thumb-1 ISA.
>
> (b) In ARMTargetLowering::LowerCall, put another constraint like
> number of arguments, as a proxy for register pressure for Thumb-1, but
> that's bound to trip another cases.
>
> (c) Give higher priority to allocate vrit reg used for indirect calls
> ? However, if that
> results in spilling of some other register, it would defeat the
> purpose of saving code-size. I suppose ideally we want to trigger the
> heuristic of using indirect call only when we know beforehand that it
> will not result in spilling. But I am not sure if it's possible to
> estimate that during isel ?
>
> I would be grateful for suggestions on how to proceed further.
ping ?

Thanks,
Prathamesh
>
> [1] https://github.com/intel/tinycrypt/blob/master/lib/source/ecc_dh.c#L139
>
> Thanks,
> Prathamesh
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