Index: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1747,6 +1747,32 @@
       return true;
   }
 
+  // Try to evaluate the value range of the source integer. If all the value is
+  // within the value range of 2^DestNumSigBits, then the value can be
+  // accurately represented.
+  PHINode *Phi = dyn_cast<PHINode>(Src);
+  if (Phi && (Phi->getNumIncomingValues() == 2) &&
+      Phi->getParent() == Phi->getIncomingBlock(1) && Phi->getNumUses() == 2 &&
+      (I.getParent() == Phi->getIncomingBlock(1))) {
+    auto *Previous = dyn_cast<ConstantInt>(
+        Phi->getIncomingValueForBlock(Phi->getIncomingBlock(0)));
+    auto *Next = dyn_cast<Instruction>(
+        Phi->getIncomingValueForBlock(Phi->getIncomingBlock(1)));
+
+    if (!Previous || !Next || Next->getOpcode() != Instruction::Add)
+      return false;
+
+    int64_t InitValue = Previous->getSExtValue();
+    ICmpInst *CmpInst = dyn_cast<ICmpInst>(Next->use_begin()->getUser());
+    if (!CmpInst || !dyn_cast<ConstantInt>(CmpInst->getOperand(1)))
+      return false;
+    int64_t EndValue =
+        cast<ConstantInt>(CmpInst->getOperand(1))->getSExtValue();
+
+    return ((InitValue >> DestNumSigBits) == 0) &&
+           ((EndValue >> DestNumSigBits) == 0);
+  }
+
   // TODO:
   // Try harder to find if the source integer type has less significant bits.
   // For example, compute number of sign bits or compute low bit mask.
Index: llvm/test/Transforms/InstCombine/fptrunc.ll
===================================================================
--- llvm/test/Transforms/InstCombine/fptrunc.ll
+++ llvm/test/Transforms/InstCombine/fptrunc.ll
@@ -190,3 +190,74 @@
   %r = fptrunc float %x to half
   ret half %r
 }
+
+@array = dso_local global [101 x i32] zeroinitializer, align 4
+; Positive test - intermediate 16777215 (= 1 << 24 - 1) rounding in float type.
+define void @foo_max () {
+; CHECK-LABEL: @foo_max(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[INDVAR_PHI:%.*]] = phi i32 [ 16777215, [[ENTRY:%.*]] ], [ [[DEC_INT:%.*]], [[FOR_BODY]] ]
+; CHECK-NEXT:    [[IDX:%.*]] = sext i32 [[INDVAR_PHI]] to i64
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [101 x i32], [101 x i32]* @array, i64 0, i64 [[IDX]]
+; CHECK-NEXT:    store i32 [[INDVAR_PHI]], i32* [[ARRAYIDX]], align 4
+; CHECK-NEXT:    [[DEC_INT]] = add nsw i32 [[INDVAR_PHI]], -1
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp eq i32 [[DEC_INT]], 0
+; CHECK-NEXT:    br i1 [[CMP_NOT]], label [[CLEANUP:%.*]], label [[FOR_BODY]]
+; CHECK:       cleanup:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.body
+
+for.body:                                ; preds = %entry, %for.body
+  %indvar.phi = phi i32 [ 16777215, %entry ], [ %dec.int, %for.body ]
+  %indvar.fp = sitofp i32 %indvar.phi to float
+  %indvar.si = fptosi float %indvar.fp to i32
+  %idx = sext i32 %indvar.si to i64
+  %arrayidx = getelementptr inbounds [101 x i32], [101 x i32]* @array, i64 0, i64 %idx
+  store i32 %indvar.si, i32* %arrayidx, align 4
+  %dec.int = add nsw i32 %indvar.phi, -1
+  %cmp = icmp ugt i32 %dec.int, 0
+  br i1 %cmp, label %for.body, label %cleanup
+
+cleanup:                                 ; preds = %for.body
+  ret void
+}
+
+; Negative test - intermediate 16777216 (= 1 << 24) rounding in float type.
+define void @float_overflow () {
+; CHECK-LABEL: @float_overflow(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[INDVAR_PHI:%.*]] = phi i32 [ 16777216, [[ENTRY:%.*]] ], [ [[DEC_INT:%.*]], [[FOR_BODY]] ]
+; CHECK-NEXT:    [[INDVAR_FP:%.*]] = sitofp i32 [[INDVAR_PHI]] to float
+; CHECK-NEXT:    [[INDVAR_SI:%.*]] = fptosi float [[INDVAR_FP]] to i32
+; CHECK-NEXT:    [[IDX:%.*]] = sext i32 [[INDVAR_SI]] to i64
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [101 x i32], [101 x i32]* @array, i64 0, i64 [[IDX]]
+; CHECK-NEXT:    store i32 [[INDVAR_SI]], i32* [[ARRAYIDX]], align 4
+; CHECK-NEXT:    [[DEC_INT]] = add nsw i32 [[INDVAR_PHI]], -1
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp eq i32 [[DEC_INT]], 0
+; CHECK-NEXT:    br i1 [[CMP_NOT]], label [[CLEANUP:%.*]], label [[FOR_BODY]]
+; CHECK:       cleanup:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.body
+
+for.body:                                ; preds = %entry, %for.body
+  %indvar.phi = phi i32 [ 16777216, %entry ], [ %dec.int, %for.body ]
+  %indvar.fp = sitofp i32 %indvar.phi to float
+  %indvar.si = fptosi float %indvar.fp to i32
+  %idx = sext i32 %indvar.si to i64
+  %arrayidx = getelementptr inbounds [101 x i32], [101 x i32]* @array, i64 0, i64 %idx
+  store i32 %indvar.si, i32* %arrayidx, align 4
+  %dec.int = add nsw i32 %indvar.phi, -1
+  %cmp = icmp ugt i32 %dec.int, 0
+  br i1 %cmp, label %for.body, label %cleanup
+
+cleanup:                                 ; preds = %for.body
+  ret void
+}