[LLVMdev] Cross-module function inlining

Mark Muir mark.i.r.muir at gmail.com
Wed Jan 13 13:03:17 PST 2010

On 13 Jan 2010, at 20:34, Nick Lewycky wrote:

> On 13 January 2010 12:05, Mark Muir <mark.i.r.muir at gmail.com> wrote:
> But... now there's a small problem with library calls. Symbols such as 'memset', 'malloc', etc. are being removed by global dead code elimination. They are implemented in one of the bitcode modules that are linked together (implementations are based on newlib).
> And what problems does that cause? If malloc is linked in, we're free to inline it everywhere and delete the symbol. If you meant for it to be visible to the optimizers but you don't want it to be part of the code generated for your program (ie., you'll link it against newlib later), you should mark the functions with available_externally linkage.

Sorry, I should've been more clear - the calls to _malloc and _free weren't being inlined (see example below). I'm not sure why (happens with or without -simplify-libcalls). So, the resulting .bc file from 'opt' contains live references to symbols that were in its input .bc, but for some reason it stripped them.

#include <stdlib.h>

int entries = 3;
int result;

int main()
	int i;

	// Allocate and populate the initial array.
	int* values = malloc(entries * sizeof(int));
	for (i = 0; i < entries; i ++)
		values[i] = i + 1;
	// Calculate the sum, using a dynamically allocated accumulator.
	int* acc = malloc(sizeof(int));
	*acc = 0;
	for (i = 0; i < entries; i ++)
		*acc += values[i];
	result = *acc;

	// Deallocate the memory.
	return 0;

Here's a fragment of the final machine assembly (with -O3):

	ADDCOMP out=r1 in1=r1 in2=4 conf=`ADDCOMP_SUB
	WMEM in=r2 in_addr=r1 conf=`WMEM_SI
	CONST_16B out=r3 conf=12
	JUMP nl_out=r2/*RA*/ addr_in=&_malloc conf=`JUMP_ALWAYS_ABS // Call

In case this is important, here is the relevant declarations from the 'stdlib.h' that is in use:

_PTR    _EXFUN(malloc,(size_t __size));
_VOID   _EXFUN(free,(_PTR));


#define _PTR            void *
#define _EXFUN(name, proto)             name proto

and from 'newlib.c':

void *
malloc (size_t sz)

i.e. They look like any other function call, which is why I suspect it has something to do with special behaviour given to built-ins.

> Alternately, if you wanted malloc, memset and friends to be externally visible (compiled as part of your program and dlsym'able), you could create a public api file which contains a one per line list of the names of the functions that may not be marked internal linkage by internalize. Pass that in to opt with -internalize-public-api-file filename ...other flags...

I saw that. I was thinking of only using that option as a last resort, due to maintainability.

> I guess I need help with the concept of built-ins, and what code is related to them in the Clang driver and back-end.


- Mark

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