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

[via llvm-commits]

================
@@ -3214,6 +3259,109 @@ bool SimplifyCFGOpt::speculativelyExecuteBB(BranchInst *BI,
   BB->splice(BI->getIterator(), ThenBB, ThenBB->begin(),
              std::prev(ThenBB->end()));
 
+  // If the target supports conditional faulting,
+  // we look for the following pattern:
+  // \code
+  //   BB:
+  //     ...
+  //     %cond = icmp ult %x, %y
+  //     br i1 %cond, label %TrueBB, label %FalseBB
+  //   FalseBB:
+  //     store i32 1, ptr %q, align 4
+  //     ...
+  //   TrueBB:
+  //     %maskedloadstore = load i32, ptr %b, align 4
+  //     store i32 %maskedloadstore, ptr %p, align 4
+  //     ...
+  // \endcode
+  //
+  // and transform it into:
+  //
+  // \code
+  //   BB:
+  //     ...
+  //     %cond = icmp ult %x, %y
+  //     %maskedloadstore = cload i32, ptr %b, %cond
+  //     cstore i32 %maskedloadstore, ptr %p, %cond
+  //     cstore i32 1, ptr %q, ~%cond
+  //     br i1 %cond, label %TrueBB, label %FalseBB
+  //   FalseBB:
+  //     ...
+  //   TrueBB:
+  //     ...
+  // \endcode
+  //
+  // where cload/cstore are represented by llvm.masked.load/store intrinsics,
+  // e.g.
+  //
+  // \code
+  //   %vcond = bitcast i1 %cond to <1 x i1>
+  //   %v0 = call <1 x i32> @llvm.masked.load.v1i32.p0
+  //                         (ptr %b, i32 4, <1 x i1> %vcond, <1 x i32> poison)
+  //   %maskedloadstore = bitcast <1 x i32> %v0 to i32
+  //   call void @llvm.masked.store.v1i32.p0
+  //                          (<1 x i32> %v0, ptr %p, i32 4, <1 x i1> %vcond)
+  //   %cond.not = xor i1 %cond, true
+  //   %vcond.not = bitcast i1 %cond.not to <1 x i>
+  //   call void @llvm.masked.store.v1i32.p0
+  //              (<1 x i32> <i32 1>, ptr %q, i32 4, <1x i1> %vcond.not)
+  // \endcode
+  //
+  // So we need to turn hoisted load/store into cload/cstore.
+  auto &Context = BI->getParent()->getContext();
+  auto *VCondTy = FixedVectorType::get(Type::getInt1Ty(Context), 1);
+  auto *Cond = BI->getOperand(0);
+  Value *Mask = nullptr;
+  // Construct the condition if needed.
+  if (!SpeculatedConditionalLoadsStores.empty()) {
+    IRBuilder<> Builder(SpeculatedConditionalLoadsStores.back());
+    if (Invert)
+      Mask = Builder.CreateBitCast(
+          Builder.CreateXor(Cond, ConstantInt::getTrue(Context)), VCondTy);
+    else
+      Mask = Builder.CreateBitCast(Cond, VCondTy);
----------------
DianQK wrote:

```suggestion
    Mask = Builder.CreateBitCast(Invert ? Builder.CreateXor(Cond, ConstantInt::getTrue(Context)) : Cond, VCondTy);
```

Is it possible to avoid creating a null pointer?

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