[LLVMdev] Bug? Coalescing & Updating Subreg Intervals

[David Greene]

I have a question about what is going on at line 754 of 
SimpleRegisterCoalescing.cpp.  The comment says we are
updating the live intervals for subregisters.  This happens when
we coalesce to a physical register.

Now, I read that as, "merge in the range information from the
eliminated live interval to the subregister live interval," but that
appears to not be what happens.

In my case, I have the following two intervals which are being
coalesced:

%reg1026,0 = [458,5168:0 [0])[5180,15248:0 [0])  0 at 458

%reg15,inf = [0,10:0 [0])[10,458:87 [0])[938,942:1 [0])[942,943:2 [0])
[962,966:3 [0])[966,967:4 [0])[1242,1246:5 [0])[1246,1247:6 [0])[1266,1270:7 
[0])[1270,1271:8 [0])[1578,1582:9 [0])[1582,1583:10 [0])[1602,1606:11 [0])
[1606,1607:12 [0])[2314,2318:13 [0])[2318,2319:14 [0])[2338,2342:15 [0])
[2342,2343:16 [0])[3366,3370:17 [0])[3370,3371:18 [0])[3390,3394:19 [0])
[3394,3395:20 [0])[3642,3646:21 [0])[3646,3647:22 [0])[3666,3670:23 [0])
[3670,3671:24 [0])[3942,3946:25 [0])[3946,3947:26 [0])[3966,3970:27 [0])
[3970,3971:28 [0])[6098,6102:29 [0])[6102,6103:30 [0])[6126,6130:31 [0])
[6130,6131:32 [0])[6390,6394:33 [0])[6394,6395:34 [0])[6418,6422:35 [0])
[6422,6423:36 [0])[6714,6718:37 [0])[6718,6719:38 [0])[6742,6746:39 [0])
[6746,6747:40 [0])[7434,7438:41 [0])[7438,7439:42 [0])[7462,7466:43 [0])
[7466,7467:44 [0])[8446,8450:45 [0])[8450,8451:46 [0])[8474,8478:47 [0])
[8478,8479:48 [0])[8710,8714:49 [0])[8714,8715:50 [0])[8738,8742:51 [0])
[8742,8743:52 [0])[8998,9002:53 [0])[9002,9003:54 [0])[9022,9026:55 [0])
[9026,9027:56 [0])[10490,10494:57 [0])[10494,10495:58 [0])[10518,10522:59 
[0])[10522,10523:60 [0])[10782,10786:61 [0])[10786,10787:62 [0])
[10810,10814:63 [0])[10814,10815:64 [0])[11106,11110:65 [0])[11110,11111:66 
[0])[11134,11138:67 [0])[11138,11139:68 [0])[11826,11830:69 [0])
[11830,11831:70 [0])[11854,11858:71 [0])[11858,11859:72 [0])[12838,12842:73 
[0])[12842,12843:74 [0])[12866,12870:75 [0])[12870,12871:76 [0])
[13102,13106:77 [0])[13106,13107:78 [0])[13130,13134:79 [0])[13134,13135:80 
[0])[13390,13394:81 [0])[13394,13395:82 [0])[13414,13418:83 [0])
[13418,13419:84 [0])[15350,15374:88 [0])[15374,15382:85 [0])[15382,15383:86 
[0])  0 at 0-(10) 1 at 938-(942) 2 at 942-(943) 3 at 962-(966) 4 at 966-(967) 5 at 1242-(1246) 
6 at 1246-(1247) 7 at 1266-(1270) 8 at 1270-(1271) 9 at 1578-(1582) 10 at 1582-(1583) 
11 at 1602-(1606) 12 at 1606-(1607) 13 at 2314-(2318) 14 at 2318-(2319) 15 at 2338-(2342) 
16 at 2342-(2343) 17 at 3366-(3370) 18 at 3370-(3371) 19 at 3390-(3394) 20 at 3394-(3395) 
21 at 3642-(3646) 22 at 3646-(3647) 23 at 3666-(3670) 24 at 3670-(3671) 25 at 3942-(3946) 
26 at 3946-(3947) 27 at 3966-(3970) 28 at 3970-(3971) 29 at 6098-(6102) 30 at 6102-(6103) 
31 at 6126-(6130) 32 at 6130-(6131) 33 at 6390-(6394) 34 at 6394-(6395) 35 at 6418-(6422) 
36 at 6422-(6423) 37 at 6714-(6718) 38 at 6718-(6719) 39 at 6742-(6746) 40 at 6746-(6747) 
41 at 7434-(7438) 42 at 7438-(7439) 43 at 7462-(7466) 44 at 7466-(7467) 45 at 8446-(8450) 
46 at 8450-(8451) 47 at 8474-(8478) 48 at 8478-(8479) 49 at 8710-(8714) 50 at 8714-(8715) 
51 at 8738-(8742) 52 at 8742-(8743) 53 at 8998-(9002) 54 at 9002-(9003) 55 at 9022-(9026) 
56 at 9026-(9027) 57 at 10490-(10494) 58 at 10494-(10495) 59 at 10518-(10522) 
60 at 10522-(10523) 61 at 10782-(10786) 62 at 10786-(10787) 63 at 10810-(10814) 
64 at 10814-(10815) 65 at 11106-(11110) 66 at 11110-(11111) 67 at 11134-(11138) 
68 at 11138-(11139) 69 at 11826-(11830) 70 at 11830-(11831) 71 at 11854-(11858) 
72 at 11858-(11859) 73 at 12838-(12842) 74 at 12842-(12843) 75 at 12866-(12870) 
76 at 12870-(12871) 77 at 13102-(13106) 78 at 13106-(13107) 79 at 13130-(13134) 
80 at 13134-(13135) 81 at 13390-(13394) 82 at 13394-(13395) 83 at 13414-(13418) 
84 at 13418-(13419) 85 at 15374-(15382) 86 at 15382-(15383) 87@? 88@?

MergeInClobberRanges comes along and sees that [458,5168:0 [0])
from %reg1026 overlaps [938,942:1 [0]) from %reg15.  It then proceeds
to add [458,938:89 [0]) to %reg15.

It then moves on to the next LiveRange of %reg1026.

Huh?  What happened to the rest of [458,5168:0 [0])?  Doesn't the subreg live
interval need to contain this whole LiveRange?

Or am I not understanding what this part of the coalescing code does?

I discovered this through an assert I put into some of my own code.  I want to
know if that assert is bogus or if there's a bug here.

Thanks!

                                            -Dave