[IR] Don't assume readnone/readonly intrinsics are willreturn

This removes our "temporary" hack to assume that readnone/readonly
intrinsics are also willreturn. An explicit willreturn annotation,
usually via default intrinsic attributes, is now required.

Differential Revision: https://reviews.llvm.org/D137630

Author: nikic

llvm/lib/IR/Instruction.cpp

@@ -744,11 +744,7 @@ bool Instruction::willReturn() const {
     return !SI->isVolatile();
 
   if (const auto *CB = dyn_cast<CallBase>(this))
-    // FIXME: Temporarily assume that all side-effect free intrinsics will
-    // return. Remove this workaround once all intrinsics are appropriately
-    // annotated.
-    return CB->hasFnAttr(Attribute::WillReturn) ||
-           (isa<IntrinsicInst>(CB) && CB->onlyReadsMemory());
+    return CB->hasFnAttr(Attribute::WillReturn);
   return true;
 }
 

llvm/lib/Transforms/Utils/Local.cpp

@@ -58,6 +58,7 @@
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/IntrinsicsWebAssembly.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/Metadata.h"
@@ -444,8 +445,24 @@ bool llvm::wouldInstructionBeTriviallyDead(Instruction *I,
     if (isRemovableAlloc(CB, TLI))
       return true;
 
-  if (!I->willReturn())
-    return false;
+  if (!I->willReturn()) {
+    auto *II = dyn_cast<IntrinsicInst>(I);
+    if (!II)
+      return false;
+
+    // TODO: These intrinsics are not safe to remove, because this may remove
+    // a well-defined trap.
+    switch (II->getIntrinsicID()) {
+    case Intrinsic::wasm_trunc_signed:
+    case Intrinsic::wasm_trunc_unsigned:
+    case Intrinsic::ptrauth_auth:
+    case Intrinsic::ptrauth_resign:
+    case Intrinsic::ptrauth_sign:
+      return true;
+    default:
+      return false;
+    }
+  }
 
   if (!I->mayHaveSideEffects())
     return true;