Re: [Haskell-cafe] redundant loads and saves in code generated for recursive functions?

30 Jul 2014

      Hi Jyotirmoy,

I didn't read your assembly carefully, but it sounds similar to
https://ghc.haskell.org/trac/ghc/ticket/8905, which is not fixed yet.

On Wed, Jul 30, 2014 at 12:03 PM, Jyotirmoy Bhattacharya
 wrote:
...
On reading this again I realise that I got the order of loads and stores
wrong. The arguments are being stored on entering the function and loaded
before the call. But still, is there a chance of eliminating this
redundancy?
Jyotirmoy
On Wed, Jul 30, 2014 at 1:54 PM, Jyotirmoy Bhattacharya
 wrote:
...
Dear All,
I am new to Haskell so please forgive me if I am asking about something
already well-understood.
I was trying to understand the performance of my Haskell program compiled
with the LLVM backend. I used -ddump-llvm to dump the LLVM assembly and then
ran llc -O3 on the resulting file to look at the native assembly.
One of the generated function starts off with
s5BH_info:                              # @s5BH_info
# BB#0:
        subq    $208, %rsp
        movq    %r13, 200(%rsp)
        movq    %rbp, 192(%rsp)
        movq    %r12, 184(%rsp)
        movq    %rbx, 176(%rsp)
        movq    %r14, 168(%rsp)
        movq    %rsi, 160(%rsp)
        movq    %rdi, 152(%rsp)
        movq    %r8, 144(%rsp)
        movq    %r9, 136(%rsp)
        movq    %r15, 128(%rsp)
        movss   %xmm1, 124(%rsp)
        movss   %xmm2, 120(%rsp)
        movss   %xmm3, 116(%rsp)
        movss   %xmm4, 112(%rsp)
        movsd   %xmm5, 104(%rsp)
        movsd   %xmm6, 96(%rsp)
At some point down the line the function makes a tail call to itself and
this is the code generated
        movq    %r14, 168(%rsp)
        movq    200(%rsp), %r13
        movq    192(%rsp), %rbp
        movq    184(%rsp), %r12
        movq    176(%rsp), %rbx
        movq    128(%rsp), %r15
        movsd   104(%rsp), %xmm5
        addq    $208, %rsp
        jmp     s5BH_info
So it looks like some values are being moved from registers to the stack
only to be immediately moved from the stack to the register on entry to the
function. It should be possible to eliminate both the load and the stores.
Is this behaviour due to LLVM or GHC? If it is GHC, it this an
optimization a newcomer can attempt to implement or are there deep issues
here?
Jyotirmoy Bhattacharya
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Re: [Haskell-cafe] redundant loads and saves in code generated for recursive functions?

Johan Tibell