llvm dev - Feb 2007 - [LLVMdev] Unused malloc/free don't get optimized

Hi, i have some code that allocate some memory, store the pointer to a
variable, read it back and deallocates it, like this:

int %main(int %argc, ubyte** %argv)
{
%c_19 = alloca ubyte*
%malloc_206  = malloc ubyte, uint 10
store ubyte* %malloc_206, ubyte** %c_19
%tmp_207 = load ubyte** %c_19
free ubyte* %tmp_207

ret int 0
}

i expected the optimized to remove everything, but after running it
the code i get is:

int %main(int %argc, ubyte** %argv) {
	%malloc_206 = malloc [10 x ubyte]
	%malloc_206.sub = getelementptr [10 x ubyte]* %malloc_206, int 0, int 0
	free ubyte* %malloc_206.sub
	ret int 0
}

Why didn't he optimized it out? and where did that getelementptr came from?
I have the feeling that i'm missing something.

On Tue, 13 Feb 2007, Nicola Lugato wrote:
> Hi, i have some code that allocate some memory, store the pointer to a
> variable, read it back and deallocates it, like this:
ok
> i expected the optimized to remove everything, but after running it
> the code i get is:
>
> int %main(int %argc, ubyte** %argv) {
> 	%malloc_206 = malloc [10 x ubyte]
> 	%malloc_206.sub = getelementptr [10 x ubyte]* %malloc_206, int 0, int 0
> 	free ubyte* %malloc_206.sub
> 	ret int 0
> }
>
> Why didn't he optimized it out?
This looks like a simple missed optimization.
> and where did that getelementptr came from?
This is just passing a pointer to the first byte of the array into the 
free instruction.  This has the same semantics as freeing the pointer to 
the array, since they are the same value.
> I have the feeling that i'm missing something.
Nope, I don't think so.  Please feel free to file an enhancement request 
for us to catch this case!

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

I've made some other test and it looks like it don't remove even
simple malloc/free couple. Maybe there's something wrong in the way i
use the opt command. How can i tell him to optimize at best, whitout
specifing each pass? I'm not using c/c++ frontend, just the assembler.

Thanks in advance

On 2/14/07, Chris Lattner <sabre at nondot.org>
wrote:
> On Tue, 13 Feb 2007, Nicola Lugato wrote:
> > Hi, i have some code that allocate some memory, store the pointer to a
> > variable, read it back and deallocates it, like this:
>
> ok
>
> > i expected the optimized to remove everything, but after running it
> > the code i get is:
> >
> > int %main(int %argc, ubyte** %argv) {
> >       %malloc_206 = malloc [10 x ubyte]
> >       %malloc_206.sub = getelementptr [10 x ubyte]* %malloc_206, int
0, int 0
> >       free ubyte* %malloc_206.sub
> >       ret int 0
> > }
> >
> > Why didn't he optimized it out?
>
> This looks like a simple missed optimization.
>
> > and where did that getelementptr came from?
>
> This is just passing a pointer to the first byte of the array into the
> free instruction.  This has the same semantics as freeing the pointer to
> the array, since they are the same value.
>
> > I have the feeling that i'm missing something.
>
> Nope, I don't think so.  Please feel free to file an enhancement
request
> for us to catch this case!
>
> -Chris
>
> --
> http://nondot.org/sabre/
> http://llvm.org/
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