[llvm] [DSE] Stop double-counting the dead slice offset in shortenAssignment (PR #216227)

[Orlando Cazalet-Hyams via llvm-commits]

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@@ -16,22 +16,37 @@
 ;;   __builtin_memset(local2, 8, 16); // local2:  0-128  (0, 128)
 ;;   esc(local2);
 ;; }
+;; void shortenEndPartial() {
+;;   char local3[80];                      //         bits    frag
+;;   __builtin_memset(local3 + 8,  0, 8);  // local3:  64-128 (64, 96)
+;;   __builtin_memset(local3 + 12, 8, 4);  // local3:  96-128 ( 96, 32)
+;;   esc(local3);
+;; }
 
 ;; The variables and intrinsics have been adjusted with by hand to test
 ;; what happens when the variable doesn't fill the whole alloca, and
 ;; when offsets are encoded with both the address component of the dbg.assign
 ;; and the address modifying DIExpression.
 
-;; DeadStoreElimination will shorten the first store in shortenEnd from [64,
-;; 192) bits to [64, 128) bits. Variable 'local' has been adjusted to be 160
-;; bits large. Check that we get an unlinked dbg.assign covering the deleted
-;; bits that overlap the dbg.assign's fagment: [128, 160) (offset=128 size=32).
+;; 'local' and 'local3' have both been adjusted to be 160 bits, so the bit
+;; ranges above are the written bytes clamped to the variable.
+
+;; In shortenEnd the killing store starts before the dead one, at local + 4
+;; against local + 8, so despite the name it takes the overwrite-begin path.
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OCHyams wrote:

> so despite the name

worth changing the name instead, if it doesn't fit?

https://github.com/llvm/llvm-project/pull/216227