[PATCH] D151945: Introduce G_OPAQUE and use it to prevent constant folding hoisted constants.

[Pierre van Houtryve via Phabricator via llvm-commits]

Pierre-vh added a comment.

Thanks a lot for making this change :)



================
Comment at: llvm/docs/GlobalISel/GenericOpcode.rst:945
+
+This operation is used as an opaque barrier to prevent constant folding. Combines
+and other transformations should not look through G_OPAQUE. These have no other
----------------
I would generalize this to "prevent optimizations such as constant folding, sinking, or other combines."


================
Comment at: llvm/include/llvm/Support/TargetOpcodes.def:335
 
+/// Constant folding barrier.
+HANDLE_TARGET_OPCODE(G_OPAQUE)
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Same, this may be used for other combines so I'd just say "Combine optimization barrier" ?


================
Comment at: llvm/include/llvm/Target/GenericOpcodes.td:1477
+
+// Prevent constant folding of the source value with any users.
+def G_OPAQUE : GenericInstruction {
----------------
ditto


================
Comment at: llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp:170-171
+      // Eliminate hints or G_OPAQUE.
+      if (isPreISelGenericOptimizationHint(MI.getOpcode()) ||
+          MI.getOpcode() == TargetOpcode::G_OPAQUE) {
+        auto [DstReg, SrcReg] = MI.getFirst2Regs();
----------------
Could `G_OPAQUE` go in `isPreISelGenericOptimizationHint` as it's also some kind of optimization hint (barrier)?


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https://reviews.llvm.org/D151945