Reapply: [Float2Int] Resolve FIXME: Pick the smallest legal type that…

… fits (#86337)

Originally reverted because of a bug in Range that is now fixed
(#86041), we can reland this commit. Tests have been added to ensure the
miscompile that caused the revert does not happen again.

llvm/include/llvm/Transforms/Scalar/Float2Int.h

@@ -44,7 +44,7 @@ class Float2IntPass : public PassInfoMixin<Float2IntPass> {
   std::optional<ConstantRange> calcRange(Instruction *I);
   void walkBackwards();
   void walkForwards();
-  bool validateAndTransform();
+  bool validateAndTransform(const DataLayout &DL);
   Value *convert(Instruction *I, Type *ToTy);
   void cleanup();
 

llvm/lib/Transforms/Scalar/Float2Int.cpp

@@ -311,7 +311,7 @@ void Float2IntPass::walkForwards() {
 }
 
 // If there is a valid transform to be done, do it.
-bool Float2IntPass::validateAndTransform() {
+bool Float2IntPass::validateAndTransform(const DataLayout &DL) {
   bool MadeChange = false;
 
   // Iterate over every disjoint partition of the def-use graph.
@@ -374,15 +374,23 @@ bool Float2IntPass::validateAndTransform() {
       LLVM_DEBUG(dbgs() << "F2I: Value not guaranteed to be representable!\n");
       continue;
     }
-    if (MinBW > 64) {
-      LLVM_DEBUG(
-          dbgs() << "F2I: Value requires more than 64 bits to represent!\n");
-      continue;
-    }
 
-    // OK, R is known to be representable. Now pick a type for it.
-    // FIXME: Pick the smallest legal type that will fit.
-    Type *Ty = (MinBW > 32) ? Type::getInt64Ty(*Ctx) : Type::getInt32Ty(*Ctx);
+    // OK, R is known to be representable.
+    // Pick the smallest legal type that will fit.
+    Type *Ty = DL.getSmallestLegalIntType(*Ctx, MinBW);
+    if (!Ty) {
+      // Every supported target supports 64-bit and 32-bit integers,
+      // so fallback to a 32 or 64-bit integer if the value fits.
+      if (MinBW <= 32) {
+        Ty = Type::getInt32Ty(*Ctx);
+      } else if (MinBW <= 64) {
+        Ty = Type::getInt64Ty(*Ctx);
+      } else {
+        LLVM_DEBUG(dbgs() << "F2I: Value requires more bits to represent than "
+                             "the target supports!\n");
+        continue;
+      }
+    }
 
     for (auto MI = ECs.member_begin(It), ME = ECs.member_end();
          MI != ME; ++MI)
@@ -489,7 +497,8 @@ bool Float2IntPass::runImpl(Function &F, const DominatorTree &DT) {
   walkBackwards();
   walkForwards();
 
-  bool Modified = validateAndTransform();
+  const DataLayout &DL = F.getParent()->getDataLayout();
+  bool Modified = validateAndTransform(DL);
   if (Modified)
     cleanup();
   return Modified;