[PATCH] D23942: Do not abort on unresolved conflicts when joining subranges

[Krzysztof Parzyszek via llvm-commits]

kparzysz added inline comments.

================
Comment at: lib/CodeGen/RegisterCoalescer.cpp:2342-2373
@@ -2341,3 +2341,34 @@
                  << '@' << LR.getValNumInfo(i)->def << '\n');
-    if (SubRangeJoin)
-      return false;
+    if (SubRangeJoin) {
+      // Unresolved subrange conflicts may remain here as a result of merging
+      // registers where subregister definitions of one register are
+      // overwritten by corresponding definitions of the other register.
+      // For example, in this case, whem coalescing vreg140 into vreg131:
+      //   704B     %vreg140<def> = ...
+      //   720B     %vreg131<def> = COPY %vreg140
+      //   736B     %vreg131:sub1<def> = COPY %vreg48
+      //   768B     %vreg131:sub2<def> = COPY %vreg23
+      //   800B     %vreg131:sub3<def> = COPY %vreg24
+      //   832B     %vreg131:sub4<def> = COPY %vreg25
+      //   864B     %vreg131:sub5<def> = COPY %vreg26
+      //   896B     %vreg131:sub6<def> = COPY %vreg40
+      //   928B     %vreg131:sub7<def> = COPY %vreg42
+      //   960B     %vreg131:sub8<def> = COPY %vreg44
+      //   976B     ... = use %vreg131
+      //   1008B    %vreg140:sub1<def> = COPY %vreg54
+      //   1040B    %vreg140:sub2<def> = COPY %vreg23
+      //   1072B    %vreg140:sub3<def> = COPY %vreg24
+      //   1104B    %vreg140:sub4<def> = COPY %vreg25
+      //   1136B    %vreg140:sub5<def> = COPY %vreg26
+      //   1168B    %vreg140:sub6<def> = COPY %vreg50
+      //   1200B    %vreg140:sub7<def> = COPY %vreg51
+      //   1232B    %vreg140:sub8<def> = COPY %vreg52
+      //   1248B    ... = use %vreg140
+      // The conflict resolution for the main live ranges of both registers
+      // can determine that the coalescing is legal and may proceed, but the
+      // subregister ranges will still contain conflicts, since the subranges
+      // will overlap between the two virtual registers. Since repeating of
+      // the resolution code below for subregisters could only result in
+      // CR_Replace, and the legality has already been determined, assume
+      // the resolution to be CR_Replace without repeating the work.
+      V.Resolution = CR_Replace;
----------------
MatzeB wrote:
> I am trying to understand this, let's look at :sub1 for example: It is defined in 736 and read in 976, in that time :sub1 of vreg140 cannot be live or the join would be invalid. After that :sub1 is written in 1008 and read in 1248, in this time vreg131:sub1 cannot be live or the join would be invalid. I don't see an overlap in the subregisters here.
Sub1 may not be the best choice here.  Consider sub2 instead: vreg140:sub2 is first defined in 704, then it is live until 1008 and subsequently overwritten in 1040.  The live subrange for it will be [704,1008)[1040,1248). This will overlap with the live range for vreg131:sub2.  The join of vreg131 and vreg140 is valid, since all the lanes written via vreg131:subN = COPY ... are never actually used via vreg140:subN.  They are considered "used" for the purpose of liveness tracking, but are not actually used by the program.  The function resolveConflicts checks specifically for that (via taintExtent and usesLanes).


Repository:
  rL LLVM

https://reviews.llvm.org/D23942