אלכס לופ' via llvm-dev
2019-Oct-27 18:23 UTC
[llvm-dev] Where and how to report an optimisation issue that doesn't cause a crash
<div dir='rtl'><div>
<div dir="rtl">
<div dir="ltr" align="left">
<div dir="ltr">"...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."</div>
<div dir="ltr"> </div>
<div dir="ltr">So what about this case <a
href="https://godbolt.org/z/xFC4Rp">https://godbolt.org/z/xFC4Rp</a>
:<br /><br /></div>
<div dir="ltr"> </div>
<div dir="ltr">
<div>struct S {</div>
<div> int a[256];</div>
<div> int b;</div>
<div>};</div>
<div> </div>
<div>int f(struct S *p, unsigned char i) {</div>
<div> if (p->b)</div>
<div> return42;</div>
<div> </div>
<div> p->a[i] = 3;</div>
<div> return p->b;</div>
<div>}</div>
<div> </div>
<div>"p->b" is re-read althoug the index "i" cannot
acces beyond the array boundary. What went wrong here?</div>
<div> </div>
<div>Thanks,</div>
<div>Alex.</div>
</div>
<br /><br /></div>
</div>
<section class="cust_msg_end"></section>
<blockquote style="margin: 0; margin-bottom: 20px; border-top: 1px solid
#e0e0e0;"><br />ב אוק׳ 27, 2019 17:47, Ivan Kosarev כתב:
<blockquote style="margin: 0; margin-bottom: 20px; border-top: 1px solid
#e0e0e0;">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>On 26/10/2019 23:39, Momchil Velikov via llvm-dev wrote:</div>
<blockquote
cite="mid:CAEjVhjRhVQ6PHA6G+O6zZtFeMf_c0jzcDR-PMDvrFXemN9+CCQ@mail.gmail.com">
<div style="font-size: 9pt; font-family:
'Calibri',sans-serif;">
<h3 style="background-color: #ffffff; font-size: 10pt; border: 1px
dotted #003333; padding: .8em;"><span style="color:
#ff6600;">CAUTION:<strong> </strong></span>This email
originated from outside of the organization. Do not click links or open
attachments unless you recognize the sender and know the content is safe. If
you suspect potential phishing or spam email, report it to <a
href="mailto:ReportSpam@accesssoftek.com">ReportSpam@accesssoftek.com</a></h3>
</div>
<div>
<div dir="ltr">
<div dir="ltr">
<div style="font-family: monospace,monospace; font-size:
small;">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.</div>
<div style="font-family: monospace,monospace; font-size:
small;"> </div>
<div style="font-family: monospace,monospace; font-size:
small;">~chill</div>
<div style="font-family: monospace,monospace; font-size:
small;"> </div>
<div style="font-family: monospace,monospace; font-size:
small;">--</div>
<div style="font-family: monospace,monospace; font-size:
small;">Compiler scrub, Arm</div>
<div style="font-family: monospace,monospace; font-size:
small;"> </div>
</div>
</div>
</div>
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<br /> </blockquote>
</blockquote>
</div></div>
Ivan Kosarev via llvm-dev
2019-Oct-28 10:33 UTC
[llvm-dev] Where and how to report an optimisation issue that doesn't cause a crash
It's just that the work on the new TBAA machinery is not completed and we do not have all the required logic for the new representation in place. On 27/10/2019 20:23, אלכס לופ' wrote:> "...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." > So what about this case https://godbolt.org/z/xFC4Rp : > > struct S { > int a[256]; > int b; > }; > int f(struct S *p, unsigned char i) { > if (p->b) > return42; > p->a[i] = 3; > return p->b; > } > "p->b" is re-read althoug the index "i" cannot acces beyond the array > boundary. What went wrong here? > Thanks, > Alex. > > > > ב אוק׳ 27, 2019 17:47, Ivan Kosarev כתב: > > Hi Momchil, > > > That seems like something that Clang can do by itself for access > > tags for index expressions with member arrays: state that they > > access the offset in the struct that corresponds to the first > > array element, so unknown indices would still conservatively > > alias between each other, but not with other struct members. > > 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. > > -- > Regards, > Ivan > > > > On 26/10/2019 23:39, Momchil Velikov via llvm-dev wrote: > > > CAUTION:**This email originated from outside of the > organization. Do not click links or open attachments > unless you recognize the sender and know the content > is safe. If you suspect potential phishing or spam > email, report it to ReportSpam at accesssoftek.com > <mailto:ReportSpam at accesssoftek.com> > > Using the shorter test case: > > struct S { > int a[3]; > int b; > }; > > int f(struct S *p, int i) { > if (p->b) > return 42; > > p->a[i] = 3; > return p->b; > } > > one can see that the the TBAA metadata loses information > about the array member: > > !4 = !{!"S", !5, i64 0, !7, i64 12} > !5 = !{!"omnipotent char", !6, i64 0} > > The "new struct path TBAA" looks better, it seems to say > "there are 12 bytes of > `int`s at offset 0 in struct S" > > (Command line was ./bin/clang -target armv7m-eabi -O2 -S > y.c -emit-llvm -Xclang > -new-struct-path-tbaa) > > > !3 = !{!4, !7, i64 12, i64 4} > !4 = !{!5, i64 16, !"S", !7, i64 0, i64 12, !7, i64 > 12, i64 4} > !5 = !{!6, i64 1, !"omnipotent char"} > !6 = !{!"Simple C/C++ TBAA"} > !7 = !{!5, i64 4, !"int"} > !8 = !{!7, !7, i64 0, i64 4} > > but then, the access tag for the store to the array > > > %arrayidx = getelementptr inbounds %struct.S, > %struct.S* %p, i32 0, i32 0, i32 %i > store i32 3, i32* %arrayidx, align 4, !tbaa !8 > > says just "it's in int" and there it still a redundant load: > > f: > ldr r2, [r0, #12] > cmp r2, #0 > itt ne > movne r0, #42 > bxne lr > movs r2, #3 > str.w r2, [r0, r1, lsl #2] > ldr r0, [r0, #12] > bx lr > > So, I manually hacked the metadata too look like: > > !8 = !{!4, !7, i64 0, i64 4} > > i.e. as if we access the first element of the array. > > Running that through `opt -O2` and `llc` yields: > > f: > ldr r2, [r0, #12] > cmp r2, #0 > iteee ne > movne r0, #42 > moveq r2, #3 > streq.w r2, [r0, r1, lsl #2] > moveq r0, #0 > bx lr > > That seems like something that Clang can do by itself for > access tags for index > expressions with member arrays: state that they access the > offset in the struct > that corresponds to the first array element, so unknown > indices would still > conservatively alias between each other, but not with > other struct members. > > Thoughts? Pitfalls? I may give it a shot. > ~chill > -- > Compiler scrub, Arm > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20191028/8938cfce/attachment.html>