[SimplifyCFG] Fold `llvm.guard(false)` to unreachable

[sanjoy]

This is incorrect, please ignore. I'll upload a new patch soon.

lib/Transforms/Utils/Local.cpp

do {
 
     // Do a quick scan of the basic block, turning any obviously unreachable
     // instructions into LLVM unreachable insts.  The instruction combining pass
     // canonicalizes unreachable insts into stores to null or undef.
     for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
       // Assumptions that are known to be false are equivalent to unreachable.
       // Also, if the condition is undefined, then we make the choice most
       // beneficial to the optimizer, and choose that to also be unreachable.
-      if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(BBI))
+      if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(BBI)) {
         if (II->getIntrinsicID() == Intrinsic::assume) {
           bool MakeUnreachable = false;
           if (isa<UndefValue>(II->getArgOperand(0)))
             MakeUnreachable = true;
           else if (ConstantInt *Cond =
                    dyn_cast<ConstantInt>(II->getArgOperand(0)))
             MakeUnreachable = Cond->isZero();
 
           if (MakeUnreachable) {
             // Don't insert a call to llvm.trap right before the unreachable.
             changeToUnreachable(&*BBI, false);
             Changed = true;
             break;
           }
         }
 
+        if (II->getIntrinsicID() == Intrinsic::experimental_guard)
+          // Unlike in llvm.assume, it is not "obviously profitable" for guards
+          // to treat `undef` as `false` since a guard on `undef` can still be
+          // useful for widening.
+          if (auto *CI = dyn_cast<ConstantInt>(II->getArgOperand(0)))
+            if (CI->isZero()) {
+              changeToUnreachable(II, /* UseLLVMTrap = */ false);
+              Changed = true;
+              break;
+            }
+      }
+
       if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
         if (CI->doesNotReturn()) {
           // If we found a call to a no-return function, insert an unreachable
           // instruction after it.  Make sure there isn't *already* one there
           // though.
           ++BBI;
           if (!isa<UnreachableInst>(BBI)) {
             // Don't insert a call to llvm.trap right before the unreachable.

test/Transforms/SimplifyCFG/guards.ll

+; RUN: opt -S -simplifycfg < %s | FileCheck %s
+
+declare void @llvm.experimental.guard(i1, ...)
+
+define i32 @f_0(i1 %c) {
+; CHECK-LABEL: @f_0(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: ret i32 20
+entry:
+  br i1 %c, label %true, label %false
+
+true:
+  call void(i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
+  ret i32 10
+
+false:
+  ret i32 20
+}
+
+define i32 @f_1(i1 %c) {
+; Demonstrate that we do not yet simplify a guard on undef
+
+; CHECK-LABEL: @f_1(
+; CHECK: ret i32 10
+; CHECK: ret i32 20
+
+entry:
+  br i1 %c, label %true, label %false
+
+true:
+  call void(i1, ...) @llvm.experimental.guard(i1 undef) [ "deopt"() ]
+  ret i32 10
+
+false:
+  ret i32 20
+}