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Hi Momchil,<br>
<br>
> That seems like something that Clang can do by itself for
access<br>
> tags for index expressions with member arrays: state that they<br>
> access the offset in the struct that corresponds to the first<br>
> array element, so unknown indices would still conservatively<br>
> alias between each other, but not with other struct members.<br>
<br>
Then all by-known-index array accesses would need to be encoded as
if there were accessing the first element, wouldn't they? The idea
behind the new representation was to address existing limitations by
giving the TBAA accurate information about accesses. If memory
servers me, in this specific case of an unknown index, the tag shall
refer to the whole member array, which is supposed to mean that all
and any of its elements can actually be accessed.<br>
<br>
-- <br>
Regards,<br>
Ivan<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">On 26/10/2019 23:39, Momchil Velikov
via llvm-dev wrote:<br>
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Using the shorter test case:<br>
<br>
struct S {<br>
int a[3];<br>
int b;<br>
};<br>
<br>
int f(struct S *p, int i) {<br>
if (p->b)<br>
return 42;<br>
<br>
p->a[i] = 3;<br>
return p->b;<br>
}<br>
<br>
one can see that the the TBAA metadata loses information
about the array member:<br>
<br>
!4 = !{!"S", !5, i64 0, !7, i64 12}<br>
!5 = !{!"omnipotent char", !6, i64 0}<br>
<br>
The "new struct path TBAA" looks better, it seems to say
"there are 12 bytes of<br>
`int`s at offset 0 in struct S"<br>
<br>
(Command line was ./bin/clang -target armv7m-eabi -O2 -S
y.c -emit-llvm -Xclang<br>
-new-struct-path-tbaa)<br>
<br>
<br>
!3 = !{!4, !7, i64 12, i64 4}<br>
!4 = !{!5, i64 16, !"S", !7, i64 0, i64 12, !7, i64
12, i64 4}<br>
!5 = !{!6, i64 1, !"omnipotent char"}<br>
!6 = !{!"Simple C/C++ TBAA"}<br>
!7 = !{!5, i64 4, !"int"}<br>
!8 = !{!7, !7, i64 0, i64 4}<br>
<br>
but then, the access tag for the store to the array<br>
<br>
<br>
%arrayidx = getelementptr inbounds %struct.S,
%struct.S* %p, i32 0, i32 0, i32 %i<br>
store i32 3, i32* %arrayidx, align 4, !tbaa !8<br>
<br>
says just "it's in int" and there it still a redundant
load:<br>
<br>
f:<br>
ldr r2, [r0, #12]<br>
cmp r2, #0<br>
itt ne<br>
movne r0, #42<br>
bxne lr<br>
movs r2, #3<br>
str.w r2, [r0, r1, lsl #2]<br>
ldr r0, [r0, #12]<br>
bx lr<br>
<br>
So, I manually hacked the metadata too look like:<br>
<br>
!8 = !{!4, !7, i64 0, i64 4}<br>
<br>
i.e. as if we access the first element of the array.<br>
<br>
Running that through `opt -O2` and `llc` yields:<br>
<br>
f:<br>
ldr r2, [r0, #12]<br>
cmp r2, #0<br>
iteee ne<br>
movne r0, #42<br>
moveq r2, #3<br>
streq.w r2, [r0, r1, lsl #2]<br>
moveq r0, #0<br>
bx lr<br>
<br>
That seems like something that Clang can do by itself for
access tags for index<br>
expressions with member arrays: state that they access the
offset in the struct<br>
that corresponds to the first array element, so unknown
indices would still<br>
conservatively alias between each other, but not with
other struct members.<br>
<br>
Thoughts? Pitfalls? I may give it a shot.<br>
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<br>
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~chill</div>
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<br>
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--</div>
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Compiler scrub, Arm</div>
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