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<div class="moz-cite-prefix">On 7/6/20 11:18 AM, ramshankar r via
llvm-dev wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAPKAp6A3AJX10zYxTSWcVmZEGCWd05+MUzP2U=0_EwNm_6gi6Q@mail.gmail.com">
<pre class="moz-quote-pre" wrap="">Hi,
I have a test-case for which I seem to be getting incorrect information
from basic-aa alias-analysis. Here it is:
#include <stdio.h>
#define ARR_SIZE 4
char c[ARR_SIZE];
char *pc[ARR_SIZE];
char **ppc = pc;
int main() {
pc[0] = &c[0];
pc[1] = &c[1];
pc[2] = &c[2];
pc[3] = &c[3];
printf(" ppc: %p\n"
" pc : %p\n"
" c : %p\n"
" *pc: %p\n",
ppc, pc, c, *pc);
return 0;
}
The way I compiled it is as follows:
clang -m32 -g pointer.c -emit-llvm -S -c
opt pointer.ll -aa-pipeline=basic-aa -passes=aa-eval
-print-all-alias-modref-info -disable-output
The result from the AA-Evaluator prints this:
NoAlias: i8** getelementptr inbounds ([4 x i8*], [4 x i8*]* @pc, i32 0,
i32 0), i8*** @ppc
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<br>
<p>Isn't this talking about `&pc[0]` and `&ppc`? If so,
NoAlias seems reasonable to me.</p>
<p>I mean, you print the value of `ppc` and `pc` while the above
talks about the addresses of these.</p>
<p>(I'm a bit tired so take this as a potential answer only.)<br>
</p>
<p><br>
</p>
<p>~ Johannes</p>
<p><br>
</p>
<p><br>
</p>
<blockquote type="cite"
cite="mid:CAPKAp6A3AJX10zYxTSWcVmZEGCWd05+MUzP2U=0_EwNm_6gi6Q@mail.gmail.com">
<pre class="moz-quote-pre" wrap="">
If we run the program, the result is this:
ppc: 0x565ee028
pc : 0x565ee028
c : 0x565ee024
*pc: 0x565ee024
Basically, I would have liked if basic-aa said ppc and pc are may-aliased
to start with for this kind of usage. This is how the globals look like.
The second one: "ppc" has "pc" on the right hand side.
@pc = common dso_local global [4 x i8*] zeroinitializer, align 4, !dbg !0
@ppc = dso_local global i8** getelementptr inbounds ([4 x i8*], [4 x i8*]*
@pc, i32 0, i32 0), align 4, !dbg !6
Best regards,
Ram
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