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7762 :: Nim compiler generates strange C code

• japplegame (orginal) [2021-04-07T22:34:31+02:00] view original

  Look at this very strange code:

  N_CDECL(void, NimMain)(void) {
          void (*volatile inner)(void); // <------ volatile?!! Why?
          PreMain();
          inner = NimMainInner;
          (*inner)();
  }
  Run

  This code turns into strange asm (ARM v6, dangerous build):

  
   800002e:	4b05      	ldr	r3, [pc, #20]	; (8000044 <NimMain+0x28>)
   8000030:	9300      	str	r3, [sp, #0]
   8000032:	9b00      	ldr	r3, [sp, #0]
   8000034:	f000 f808 	bl	8000048 <NimMain+0x2c>
   ...
   
   8000044:	08000015 	.word	0x08000015
   8000048:	4718      	bx	r3
  Run

  WAT?

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• Araq (orginal) [2021-04-07T23:32:34+02:00] view original

  I don't really remember the exact reasons, but "noinline" wasn't good enough in this case, so we used a volatile function pointer. The reason is that the C compiler must not optimize out our conservative GC stack marking. None of this is required for arc/orc.

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• japplegame (orginal) [2021-04-08T00:20:50+02:00] view original

  Thank you for the clarification.

  I also found that clang generates much better asm than arm-none-eabi-gcc.

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