[GlobalISel] Compute debug location when merging stores more accurately

[asi-sc]

Summary

Originaly the loop did almost nothing as the calculated location was
overwritten on the next iteration.

[aemerson]

Thanks for fixing this, could you add a small test as well?

[asi-sc]

Add a test

[asi-sc]

In D136937#3892278, @aemerson wrote:

Thanks for fixing this, could you add a small test as well?

I added a test, but uploaded only the diff to make a stress on the change. In the test there are 4 store instructions with equal line (=9), but different column number. First two stores have column number = 1, third and fourth stores have column number = 4. That's why when we merge locations incorrectly by using only two last stores, we have column: 4 in the merged store too. However, when merging locations for all four stores, we cannot find one common column number, so we dropping this information at all.

[aemerson]

Thanks. LGTM.

llvm/lib/CodeGen/GlobalISel/LoadStoreOpt.cpp

bool LoadStoreOpt::doSingleStoreMerge(SmallVectorImpl<GStore *> &Stores) {
   // final store in the merge sequence.
   GStore *FirstStore = Stores[0];
   const unsigned NumStores = Stores.size();
   LLT SmallTy = MRI->getType(FirstStore->getValueReg());
   LLT WideValueTy =
       LLT::scalar(NumStores * SmallTy.getSizeInBits().getFixedSize());
 
   // For each store, compute pairwise merged debug locs.
-  DebugLoc MergedLoc;
-  for (unsigned AIdx = 0, BIdx = 1; BIdx < NumStores; ++AIdx, ++BIdx)
-    MergedLoc = DILocation::getMergedLocation(Stores[AIdx]->getDebugLoc(),
-                                              Stores[BIdx]->getDebugLoc());
+  DebugLoc MergedLoc = Stores.front()->getDebugLoc();
+  for (auto *Store : drop_begin(Stores))
+    MergedLoc = DILocation::getMergedLocation(MergedLoc, Store->getDebugLoc());
+
   Builder.setInstr(*Stores.back());
   Builder.setDebugLoc(MergedLoc);
 
   // If all of the store values are constants, then create a wide constant
   // directly. Otherwise, we need to generate some instructions to merge the
   // existing values together into a wider type.
   SmallVector<APInt, 8> ConstantVals;
   for (auto *Store : Stores) {

llvm/test/CodeGen/AArch64/GlobalISel/store-merging-debug.mir

bb.0:
     ; CHECK-NEXT: [[PTR_ADD1:%[0-9]+]]:_(p0) = G_PTR_ADD [[COPY]], [[C5]](s64), debug-location !DILocation(line: 11, column: 1, scope: !5)
     ; CHECK-NEXT: DBG_VALUE [[PTR_ADD1]](p0), $noreg, !9, !DIExpression(), debug-location !DILocation(line: 11, column: 1, scope: !5)
     ; CHECK-NEXT: DBG_VALUE 2, $noreg, !9, !DIExpression(), debug-location !DILocation(line: 12, column: 1, scope: !5)
     ; CHECK-NEXT: [[C6:%[0-9]+]]:_(s64) = G_CONSTANT i64 6, debug-location !DILocation(line: 13, column: 1, scope: !5)
     ; CHECK-NEXT: DBG_VALUE [[C6]](s64), $noreg, !9, !DIExpression(), debug-location !DILocation(line: 13, column: 1, scope: !5)
     ; CHECK-NEXT: [[PTR_ADD2:%[0-9]+]]:_(p0) = G_PTR_ADD [[COPY]], [[C6]](s64), debug-location !DILocation(line: 14, column: 1, scope: !5)
     ; CHECK-NEXT: DBG_VALUE [[PTR_ADD2]](p0), $noreg, !9, !DIExpression(), debug-location !DILocation(line: 14, column: 1, scope: !5)
     ; CHECK-NEXT: [[C7:%[0-9]+]]:_(s64) = G_CONSTANT i64 3940688328982532
-    ; CHECK-NEXT: G_STORE [[C7]](s64), [[COPY]](p0), debug-location !DILocation(line: 9, column: 4, scope: !5) :: (store (s64), align 2)
+    ; CHECK-NEXT: G_STORE [[C7]](s64), [[COPY]](p0), debug-location !DILocation(line: 9, scope: !5) :: (store (s64), align 2)
     ; CHECK-NEXT: DBG_VALUE 3, $noreg, !9, !DIExpression(), debug-location !DILocation(line: 15, column: 1, scope: !5)
     ; CHECK-NEXT: RET_ReallyLR debug-location !DILocation(line: 16, column: 1, scope: !5)
     %0:_(p0) = COPY $x0, debug-location !11
     DBG_VALUE %0(p0), $noreg, !9, !DIExpression(), debug-location !11
     %1:_(s16) = G_CONSTANT i16 4, debug-location !DILocation(line: 2, column: 1, scope: !5)
     DBG_VALUE %1(s16), $noreg, !9, !DIExpression(), debug-location !DILocation(line: 2, column: 1, scope: !5)
     %4:_(s16) = G_CONSTANT i16 5, debug-location !DILocation(line: 3, column: 1, scope: !5)
     DBG_VALUE %4(s16), $noreg, !9, !DIExpression(), debug-location !DILocation(line: 3, column: 1, scope: !5)