[llvm] [X86, SimplifyCFG] Support hoisting load/store with conditional faulting (Part I) (PR #96878)

[Nikita Popov via llvm-commits]

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@@ -3080,22 +3108,31 @@ bool SimplifyCFGOpt::speculativelyExecuteBB(BranchInst *BI,
 
     // Only speculatively execute a single instruction (not counting the
     // terminator) for now.
-    ++SpeculatedInstructions;
+    bool IsSafeCheapLoadStore =
+        HositLoadsStores && isSafeCheapLoadStore(&I, TTI);
+    // Not count load/store into cost if target supports conditional faulting
+    // b/c it's cheap to speculate it.
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nikic wrote:

At least as written, I don't think this reasoning makes a lot of sense. It's not like speculating a cload/cstore is *cheaper* than speculating an add, right?

And just ignoring the number entirely surely cannot be right. If you have a conditional block with 1000 stores, it does not make sense to convert all of them into cstore. I think your current implementation is going to do that?

https://github.com/llvm/llvm-project/pull/96878