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<div dir="ltr">Thanks Ivan,<br /><br /></div>
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<div dir="ltr">Any guess what side effects it can cause? I mean, this option is available today, right? Just not by default. </div>
<div dir="ltr">So how can I see if this option causes "damages"? Just by comparing the binary outputs with and without it?</div>
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<div dir="ltr">By the way, are there any plans to make this option a default one in some future release? If yes, then what release is the target?</div>
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<blockquote style="margin: 0; margin-bottom: 20px; border-top: 1px solid #e0e0e0;"><br />ב אוק׳ 29, 2019 21:20, Ivan Kosarev כתב:
<blockquote style="margin: 0; margin-bottom: 20px; border-top: 1px solid #e0e0e0;">Answering the second question, the new TBAA representation is a work in progress; it's not mature enough to be enabled by default.<br /> <br /> <br />
<div>On 29/10/2019 19:55, אלכס לופ' wrote:</div>
<blockquote cite="mid:%5E9712EEA03EA86AECAC9DA011245D35F28CB7923D@walla.com">
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<div dir="ltr" align="left">So I have a couple of question regarding the approach provided by Chill.</div>
<div dir="ltr" align="left"><br /> 1. How to prevent such memory re-reads in the future? Is there any BKM (best known method) for programming guidelines which could eliminate or reduce those re-reads?</div>
<div dir="ltr" align="left">2. What could be the downside of the flag -Xclang -new-struct-path-tbaa? Why not using it by default if it makes better aliasing analysis?</div>
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<div dir="ltr" align="left">Thanks,</div>
<div dir="ltr" align="left">Alex. </div>
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<blockquote style="margin: 0; margin-bottom: 20px; border-top: 1px solid #e0e0e0;"><br /> ב אוק׳ 28, 2019 12:33, Ivan Kosarev כתב:
<blockquote style="margin: 0; margin-bottom: 20px; border-top: 1px solid #e0e0e0;">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.<br /> <br /> <br />
<div>On 27/10/2019 20:23, אלכס לופ' wrote:</div>
<blockquote cite="mid:%5EBF2D89B43C699965E0B802DE2F492E563EFD477F@walla.com">
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<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>
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<div dir="ltr">So what about this case <a href="https://godbolt.org/z/xFC4Rp">https://godbolt.org/z/xFC4Rp</a> :<br /> </div>
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<div>struct S {</div>
<div> int a[256];</div>
<div> int b;</div>
<div>};</div>
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<div>int f(struct S *p, unsigned char i) {</div>
<div> if (p->b)</div>
<div> return42;</div>
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<div> p->a[i] = 3;</div>
<div> return p->b;</div>
<div>}</div>
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<div>"p->b" is re-read althoug the index "i" cannot acces beyond the array boundary. What went wrong here?</div>
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<div>Thanks,</div>
<div>Alex.</div>
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<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">
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<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>
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<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>
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<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>
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