[llvm-dev] [ARM] Should Use Load and Store with Register Offset

[Daniel Way via llvm-dev]

Hello Sjoerd,

Thank you for your response! I was not aware that -Oz is a closer
equivalent to GCC's -Os. I tried -Oz when compiling with clang and
confirmed that the Clang's generated assembly is equivalent to GCC for the
code snippet I posted above.

clang --target=armv6m-none-eabi -Oz -fomit-frame-pointer
memcpy_alt1:
        push    {r4, lr}
        movs    r3, #0
.LBB0_1:
        cmp     r2, r3
        beq     .LBB0_3
        ldrb    r4, [r1, r3]
        strb    r4, [r0, r3]
        adds    r3, r3, #1
        b       .LBB0_1
.LBB0_3:
        pop     {r4, pc}

On the other hand, -O2 in GCC still uses the register-offset load and store
instructions while Clang -O2 generates the same assembly as -Os:
immediate-offset (0 offset) load/store followed by incrementing the base
register addresses.
I have not tried to benchmark the Clang-generated code, it is possible that
execution time is bounded by the load and store instructions and memory
access latency. From an intuitive view, however, both GCC and Clang are
generating code with 1 load and 1 store, so if Clang inserts two additional
adds instructions, the binary size is larger, execution *could* be slower,
and there's no improvement in register utilization over GCC.

I wanted to try a couple other variants of memcpy-like functions. The
https://godbolt.org/z/d7P6rG link includes memcpy_alt2 which copies data
from src to dst starting at the high address and memcpy_silly which copies
src to dst<0-4>. Here is the behavior I have noticed from GCC and Clang.

*memcpy_alt2*

   - With -Os, GCC generates just 6 instructions. -O2 generates 7 but
   reduces branching to once per loop.
   - Clang with -Os or -O2 does a decent job of using a common register to
   offset the load and store bases. It adds some overhead, though, by
   pre-decrementing the base registers. 10 instructions generated.
   - Clang with -Oz is pathological, generating 13 instructions. It uses
   register-offset load/store instructions, but uses different registers for
   the offsets.

*memcpy_silly*

   - I created this case to see if clang would select load/store with a
   common offset register once enough load instructions were added.
   - Clang with -Os or -O2 does not seem to care about register-offset
   load/store and prefers to increment each base register address.
   - Clang with -Oz performs the optimization I want. It produces the same
   number of instructions as GCC, and avoids an issue where GCC has to re-read
   the same value from the stack each time through the loop.


I really think that, when limited to the Thumb1 ISA, register-offset load
and store instructions should be used at -Oz, -Os, and -O2 optimization
levels. Explicitly incrementing a register holding the base address seems
unnecessary when the value seems wasteful and I cannot see how it will
improve execution time in the examples I'm investigating. Id like to know
if I'm wrong in assuming that LDR Rd, [Rn, Rm] and LDR Rd, [Rn, #<imm>]
have the same execution time, but based on the Cortex-M0+ TRM they should
both require 2 clock cycles.

Best regards,

Daniel Way


On Mon, Jul 20, 2020 at 6:15 PM Sjoerd Meijer <Sjoerd.Meijer at arm.com> wrote:

> Hello Daniel,
>
> LLVM and GCC's optimisation levels are not really equivalent. In Clang,
> -Os makes a performance and code-size trade off. In GCC, -Os is minimising
> code-size, which is equivalent to -Oz with Clang. I have't looked into
> details yet, but changing -Os to -Oz in the godbolt link gives the codegen
> you're looking for?
>
> Cheers,
> Sjoerd.
> ------------------------------
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Daniel
> Way via llvm-dev <llvm-dev at lists.llvm.org>
> *Sent:* 20 July 2020 06:54
> *To:* llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
> *Subject:* [llvm-dev] [ARM] Should Use Load and Store with Register Offset
>
> Hello LLVM Community (specifically anyone working with ARM Cortex-M),
>
> While trying to compile the Newlib C library I found that Clang10 was
> generating slightly larger binaries than the libc from the prebuilt
> gcc-arm-none-eabi toolchain. I looked at a few specific functions (memcpy,
> strcpy, etc.) and noticed that LLVM does not tend to generate load/store
> instructions with a register offset (e.g. ldr Rd, [Rn, Rm] form) and
> instead prefers the immediate offset form.
>
> When copying a contiguous sequence of bytes, this results in additional
> instructions to modify the base address. https://godbolt.org/z/T1xhae
>
> void* memcpy_alt1(void* dst, const void* src, size_t len) {
>     char* save = (char*)dst;
>     for (size_t i = 0; i < len; ++i)
>         *((char*)(dst + i)) = *((char*)(src + i));
>     return save;
> }
>
> clang --target=armv6m-none-eabi -Os -fomit-frame-pointer
> memcpy_alt1:
>         push    {r4, lr}
>         cmp     r2, #0
>         beq     .LBB0_3
>         mov     r3, r0
> .LBB0_2:
>         ldrb    r4, [r1]
>         strb    r4, [r3]
>         adds    r1, r1, #1
>         adds    r3, r3, #1
>         subs    r2, r2, #1
>         bne     .LBB0_2
> .LBB0_3:
>         pop     {r4, pc}
>
> arm-none-eabi-gcc -march=armv6-m -Os
> memcpy_alt1:
>         movs    r3, #0
>         push    {r4, lr}
> .L2:
>         cmp     r3, r2
>         bne     .L3
>         pop     {r4, pc}
> .L3:
>         ldrb    r4, [r1, r3]
>         strb    r4, [r0, r3]
>         adds    r3, r3, #1
>         b       .L2
>
> Because this code appears in a loop that could be copying hundreds of
> bytes, I want to add an optimization that will prioritize load/store
> instructions with register offsets when the offset is used multiple times.
> I have not worked on LLVM before, so I'd like advice about where to start.
>
>    - The generated code is correct, just sub-optimal so is it appropriate
>    to submit a bug report?
>    - Is anyone already tackling this change or is there someone with more
>    experience interested in collaborating?
>    - Is this optimization better performed early during instruction
>    selection or late using c++ (i.e. ARMLoadStoreOptimizer.cpp)
>    - What is the potential to cause harm to other parts of the code gen,
>    specifically for other arm targets. I'm working with armv6m, but armv7m
>    offers base register updating in a single instruction. I don't want to
>    break other useful optimizations.
>
> So far, I am reading through the LLVM documentation to see where a change
> could be applied. I have also:
>
>    - Compiled with -S -emit-llvm (see Godbolt link)
>    There is an identifiable pattern where a getelementptr function is
>    followed by a load or store. When multiple getelementptr functions appear
>    with the same virtual register offset, maybe this should match a tLDRr or
>    tSTRr.
>    - Ran LLC with  --print-machineinstrs
>    It appears that tLDRBi and tSTRBi are selected very early and never
>    replaced by the equivalent t<LDRB|STRB>r instructions.
>
> Thank you,
>
> Daniel Way
>
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