[llvm] [X86, SimplifyCFG] Support hoisting load/store with conditional faulting (Part II) (PR #108812)

[Shengchen Kan via llvm-commits]

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@@ -7815,6 +7835,33 @@ bool SimplifyCFGOpt::simplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
       if (HoistCommon &&
           hoistCommonCodeFromSuccessors(BI, !Options.HoistCommonInsts))
         return requestResimplify();
+
+      if (BI && HoistLoadsStoresWithCondFaulting &&
+          Options.HoistLoadsStoresWithCondFaulting) {
+        SmallVector<Instruction *, 2> SpeculatedConditionalLoadsStores;
+        auto CanSpeculateConditionalLoadsStores = [&]() {
+          for (auto *Succ : successors(BB)) {
+            for (Instruction &I : *Succ) {
+              if (I.isTerminator()) {
+                if (I.getNumSuccessors() > 1)
+                  return false;
+                continue;
+              } else if (!isSafeCheapLoadStore(&I, TTI) ||
+                         SpeculatedConditionalLoadsStores.size() ==
+                             HoistLoadsStoresWithCondFaultingThreshold) {
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KanRobert wrote:

I think we should consider branch probability for this, e.g. `isProfitableToSpeculate`.  If A has two successors B and C, it's not profitable to execute more instructions to eliminate the branch if the branch is well-predicated and the load/store comes from the unlikely successor.

https://github.com/llvm/llvm-project/pull/108812