[llvm] [AMDGPU] Fix overlapping insert crash during rewrite-agpr-copy-mfma (PR #205962)

[Dhruva Chakrabarti via llvm-commits]

================
@@ -0,0 +1,734 @@
+# REQUIRES: asserts
+# RUN: llc -mtriple=amdgcn -mcpu=gfx950 \
+# RUN:   -start-before=register-coalescer \
+# RUN:   -stop-after=amdgpu-rewrite-agpr-copy-mfma \
+# RUN:   -verify-machineinstrs -filetype=null %s
+
+# A spill slot is stored once and reloaded far away, so the slot's LiveStacks
+# interval is discontiguous, but the unspilled replacement vreg would have to
+# live continuously across the gap. The interference check accounts for that
+# whole span, so the slot cannot be reassigned to an interfering register and is
+# left spilled. Previously the check only considered the discontiguous slot
+# interval, picked an interfering register, and LiveRegMatrix::assign hit an
+# "Overlapping insert" assertion. This test just checks that we no longer crash.
+
+--- |
+  define amdgpu_kernel void @spill_slot_discontiguous_interval(i1 %0, <16 x float> %1, <16 x float> %2, <16 x float> %3, <8 x bfloat> %4) #0 {
+    ret void
+  }
+
+  attributes #0 = { "amdgpu-wave-limiter"="true" "target-cpu"="gfx950" }
----------------
dhruvachak wrote:

Done.

https://github.com/llvm/llvm-project/pull/205962