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

[Phoebe Wang via llvm-commits]

================
@@ -1771,6 +1782,25 @@ bool SimplifyCFGOpt::hoistCommonCodeFromSuccessors(Instruction *TI,
     if (Succ->hasAddressTaken() || !Succ->getSinglePredecessor())
       return false;
 
+  auto *BI = dyn_cast<BranchInst>(TI);
+  if (BI && HoistLoadsStoresWithCondFaulting &&
+      Options.HoistLoadsStoresWithCondFaulting) {
+    SmallVector<Instruction *, 2> SpeculatedConditionalLoadsStores;
+    for (auto *Succ : successors(BB)) {
+      for (Instruction &I : drop_end(*Succ)) {
+        if (!isSafeCheapLoadStore(&I, TTI) ||
+            SpeculatedConditionalLoadsStores.size() ==
+                HoistLoadsStoresWithCondFaultingThreshold)
+          return false;
+        SpeculatedConditionalLoadsStores.push_back(&I);
+      }
+    }
+
+    if (!SpeculatedConditionalLoadsStores.empty())
----------------
phoebewang wrote:

I don't think it's necessary given the last simplifycfg is supposed to clean up single-entry-single-exit or empty blocks.
https://github.com/llvm/llvm-project/blob/main/llvm/lib/Passes/PassBuilderPipelines.cpp#L1533-L1534

If we have such pattens, we should have optimized in previous simplifycfgs:
```
$ clang -S apx.c -mapxf -mllvm -print-pipeline-passes -O1 | sed -e 's/,/\n/g' | sed -e 's/;no-[^;>]*//g' | grep ^simplifycfg
simplifycfg<bonus-inst-threshold=1;keep-loops;speculate-blocks;simplify-cond-branch>
simplifycfg<bonus-inst-threshold=1;switch-range-to-icmp;keep-loops;speculate-blocks;simplify-cond-branch>)
simplifycfg<bonus-inst-threshold=1;switch-range-to-icmp;keep-loops;speculate-blocks;simplify-cond-branch>
simplifycfg<bonus-inst-threshold=1;switch-range-to-icmp;keep-loops;speculate-blocks;simplify-cond-branch>
simplifycfg<bonus-inst-threshold=1;switch-range-to-icmp;keep-loops;speculate-blocks;simplify-cond-branch>
simplifycfg<bonus-inst-threshold=1;switch-range-to-icmp;keep-loops;speculate-blocks;simplify-cond-branch>
simplifycfg<bonus-inst-threshold=1;forward-switch-cond;switch-range-to-icmp;switch-to-lookup;hoist-common-insts;sink-common-insts;speculate-blocks;simplify-cond-branch>
simplifycfg<bonus-inst-threshold=1;switch-range-to-icmp;keep-loops;hoist-loads-stores-with-cond-faulting;speculate-blocks;simplify-cond-branch;speculate-unpredictables>)
```

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