Re: Low-level optimizations in Nim

[mratsim]

I've been playing with point 4, memory alignment.

This compiles: 
    
    
    {.pragma: align16, codegenDecl: "$# $# __attribute__((aligned(16)))".}
    
    proc newBufferArray[T: SomeNumber](N: static[int], typ: typedesc[T]): ref 
array[N, T]  {.noSideEffect.} =
      new result
      for i in 0 ..< N:
        result[i] = 0.T
    
    let x {.align16.} = newBufferArray(100, int)
    
    echo x[0]
    

However when I look at the generated C code, it seems like what is aligned is 
the ref pointer, not the actual data.

Is there a way to apply the pragma to the actual data and not the pointer.

By the way I checked and found an align pragma in 
[compiler/ast.nim](https://github.com/nim-lang/Nim/blob/8f67b90997d1c0416c4991ecec666a63339103c2/compiler/ast.nim#L878)
 and 
[compiler/pragmas.nim](https://github.com/nim-lang/Nim/blob/d7eb146d28515e9aea5ae0f98e01b09bd96b5919/compiler/pragmas.nim#L695)
 but it's not exposed (feat request 
[here](https://github.com/nim-lang/Nim/issues/1930)). Furthermore, I can 
currently use a ref object wrapper and specify the alignement as mentionned in 
[#5315](https://github.com/nim-lang/Nim/issues/5315): 
    
    
    type Foo = ref object
      bar {.align16.}: array[N,T]
    

Part of the C code: 
    
    
    ...
    L190: N_NIMCALL(NI*, newBufferArray_3aJInY9bpU4oUK9cUIQSVHDA)(void);
    L191: static N_NIMCALL(void, Marker_TY_9b0xA7syFjRd01ZWg628edQ)(void* p, NI 
op);
    ...
    L217: NI* x_Js2X9aXg71jf4IrwTQPo9b3A __attribute__((aligned(16)));
    L218: extern GcHeap_1TRH1TZMaVZTnLNcIHuNFQ gch_IcYaEuuWivYAS86vFMTS3Q;
    L219: static N_NIMCALL(void, Marker_TY_9b0xA7syFjRd01ZWg628edQ)(void* p, NI 
op) {
    L220:       NI* a;
    L221:       NI T1_;
    L222:       a = (NI*)p;
    L223:       T1_ = (NI)0;
    L224:       for (T1_ = 0; T1_ < 100; T1_++) {
    L225:       }
    L226: }
    ...
    L403: NIM_EXTERNC N_NOINLINE(void, NimMainModule)(void) {
    L404:       NimStringDesc* T1_;
    L405:       nimfr_("play_alignment", "play_alignment.nim")
    L406: nimRegisterGlobalMarker(TM_DmX0x2ZpvnN2eETjy62C0A_3);
    L407:       nimln_(10, "play_alignment.nim");
    L408:       asgnRefNoCycle((void**) (&x_Js2X9aXg71jf4IrwTQPo9b3A), 
newBufferArray_3aJInY9bpU4oUK9cUIQSVHDA());
    L409:       nimln_(12, "play_alignment.nim");
    L410:       T1_ = (NimStringDesc*)0;
    L411:       T1_ = nimIntToStr(x_Js2X9aXg71jf4IrwTQPo9b3A[(((NI) 0))- 0]);
    L411:       printf("%s\012", T1_? (T1_)->data:"nil");
    L412:       fflush(stdout);
    L413:       popFrame();
    L414: }