[llvm] [SLSR] Skipping rewriting based on liveness (PR #218470)

[Matt Arsenault via llvm-commits]

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@@ -309,6 +309,17 @@ unsigned GCNTTIImpl::getNumberOfRegisters(unsigned RCID) const {
   return 4;
 }
 
+std::optional<unsigned> GCNTTIImpl::getRegisterBudget(const Function &F) const {
+  // Report the VGPR budget implied by the occupancy F is compiled for. Callers
+  // comparing a single lumped pressure number against this should be
+  // conservative on the SGPR side, which is intentional.
+  // Leave the allocator some slack.
+  constexpr unsigned SafeMargin = 8;
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arsenm wrote:

I don't understand this margin 

https://github.com/llvm/llvm-project/pull/218470