[llvm] [MachineSink] Add option for aggressive loop sinking (PR #117247)

[Jeffrey Byrnes via llvm-commits]

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@@ -1574,6 +1615,149 @@ bool MachineSinking::hasStoreBetween(MachineBasicBlock *From,
   return HasAliasedStore;
 }
 
+/// Copy paste from DeadMachineInstructionElimImpl
+
+bool MachineSinking::isDead(const MachineInstr *MI) const {
+  // Instructions without side-effects are dead iff they only define dead regs.
+  // This function is hot and this loop returns early in the common case,
+  // so only perform additional checks before this if absolutely necessary.
+  for (const MachineOperand &MO : MI->all_defs()) {
+    Register Reg = MO.getReg();
+    if (Reg.isPhysical()) {
+      return false;
+    } else {
+      if (MO.isDead()) {
+#ifndef NDEBUG
+        // Basic check on the register. All of them should be 'undef'.
+        for (auto &U : MRI->use_nodbg_operands(Reg))
+          assert(U.isUndef() && "'Undef' use on a 'dead' register is found!");
+#endif
+        continue;
+      }
+      for (const MachineInstr &Use : MRI->use_nodbg_instructions(Reg)) {
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jrbyrnes wrote:

Maybe it's supposed to work but doesn't? Calling hasOneNonDBGUser on the %8 def in `%8:sreg_64 = S_MOV_B64 -1` returns true for the single user `%9:sreg_64 = S_AND_B64 $exec, %8:sreg_64, implicit-def dead $scc`
 https://godbolt.org/z/MMMnaTxdP  -- !(MRI->hasAtMostUserInstrs(Reg,0) seems to have the behavior we want though.

https://github.com/llvm/llvm-project/pull/117247