[llvm] [CodeGen] Refactor DeadMIElim isDead and GISel isTriviallyDead (PR #105956)

[Tobias Stadler via llvm-commits]

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@@ -119,8 +106,18 @@ bool DeadMachineInstructionElimImpl::isDead(const MachineInstr *MI) const {
     }
   }
 
-  // If there are no defs with uses, the instruction is dead.
-  return true;
+  // Technically speaking inline asm without side effects and no defs can still
+  // be deleted. But there is so much bad inline asm code out there, we should
+  // let them be.
+  if (MI->isInlineAsm())
+    return false;
+
+  // FIXME: See issue #105950 for why LIFETIME markers are considered dead here.
+  if (MI->isLifetimeMarker())
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tobias-stadler wrote:

Yes, I want to fix this properly, but I wanted to keep this NFC except for the compile-time improvements. I think for now it's better to make this sketchy behavior explicit in DeadMIElim rather than silently dropping lifetime markers by pretending they are safe to move, which makes little sense. I want to add lifetime markers to MachineInstr::isPosition, which makes them unsafe to move. This will get rid of the ugly opposite check.

https://github.com/llvm/llvm-project/pull/105956