[IndVars] Add check of loop invariant for trunc instructions (#71072)

The same idea as in 34d380e, but considering
truncation instructions.
Improvement for #59633.

Author: markoshorro

llvm/lib/Transforms/Utils/SimplifyIndVar.cpp

@@ -909,8 +909,9 @@ void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
     if (replaceIVUserWithLoopInvariant(UseInst))
       continue;
 
-    // Go further for the bitcast ''prtoint ptr to i64'
-    if (isa<PtrToIntInst>(UseInst))
+    // Go further for the bitcast 'prtoint ptr to i64' or if the cast is done
+    // by truncation
+    if ((isa<PtrToIntInst>(UseInst)) || (isa<TruncInst>(UseInst)))
       for (Use &U : UseInst->uses()) {
         Instruction *User = cast<Instruction>(U.getUser());
         if (replaceIVUserWithLoopInvariant(User))

llvm/test/Transforms/IndVarSimplify/X86/eliminate-trunc.ll

@@ -503,8 +503,7 @@ define void @test_11() {
 ; CHECK:       bb4:
 ; CHECK-NEXT:    br label [[BB6]]
 ; CHECK:       bb5:
-; CHECK-NEXT:    [[_TMP24:%.*]] = icmp slt i16 poison, 0
-; CHECK-NEXT:    br i1 [[_TMP24]], label [[BB5:%.*]], label [[BB5]]
+; CHECK-NEXT:    br i1 poison, label [[BB5:%.*]], label [[BB5]]
 ; CHECK:       bb6:
 ; CHECK-NEXT:    br i1 false, label [[BB1]], label [[BB7:%.*]]
 ; CHECK:       bb7:

llvm/test/Transforms/IndVarSimplify/casted-trunc.ll

@@ -0,0 +1,28 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=indvars -S | FileCheck %s
+
+declare void @foo(i16 noundef)
+
+; Function Attrs: mustprogress noreturn uwtable
+define void @bar(i64 noundef %ptr) {
+; CHECK-LABEL: @bar(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = trunc i64 [[PTR:%.*]] to i4
+; CHECK-NEXT:    [[TMP1:%.*]] = zext i4 [[TMP0]] to i16
+; CHECK-NEXT:    br label [[WHILE_BODY:%.*]]
+; CHECK:       while.body:
+; CHECK-NEXT:    tail call void @foo(i16 noundef signext [[TMP1]])
+; CHECK-NEXT:    br label [[WHILE_BODY]]
+;
+entry:
+  br label %while.body
+
+while.body:                                       ; preds = %entry, %while.body
+  %0 = phi i64 [ %ptr, %entry ], [ %add.ptr, %while.body ]
+  %1 = trunc i64 %0 to i16
+  %and = and i16 %1, 15                           ; loop invariant
+  tail call void @foo(i16 noundef signext %and)
+  %add.ptr = add nsw i64 %0,  16
+  br label %while.body
+}
+