Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -3249,6 +3249,18 @@
         }
       }
     }
+
+    Value *X;
+    // Sign-bit checks can be optimized to true/false after unsigned
+    // floating-point casts:
+    // icmp slt (bitcast (uitofp X)),  0 --> false
+    // icmp sgt (bitcast (uitofp X)), -1 --> true
+    if (match(LHS, m_BitCast(m_UIToFP(m_Value(X))))) {
+      if (Pred == ICmpInst::ICMP_SLT && match(RHS, m_Zero()))
+        return ConstantInt::getFalse(ITy);
+      if (Pred == ICmpInst::ICMP_SGT && match(RHS, m_AllOnes()))
+        return ConstantInt::getTrue(ITy);
+    }
   }
 
   // icmp eq|ne X, Y -> false|true if X != Y
Index: test/Transforms/InstSimplify/cast-unsigned-icmp-cmp-0.ll
===================================================================
--- test/Transforms/InstSimplify/cast-unsigned-icmp-cmp-0.ll
+++ test/Transforms/InstSimplify/cast-unsigned-icmp-cmp-0.ll
@@ -1,6 +1,9 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -instcombine -instsimplify -S | FileCheck %s
 
+; NOTE: we run instcombine first,
+; to make sure that it does *NOT* drop uitofp+bitcast here.
+
 ; target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 
 ; This is related to https://bugs.llvm.org/show_bug.cgi?id=36682
@@ -10,12 +13,11 @@
 ;   * slt i32 %b, 0  -> false
 ;   * sgt i32 %b, -1 -> true
 
+; FIXME: icmp slt does not handle undef elements in vectors.
+
 define i1 @i32_cast_cmp_slt_int_0_uitofp_float(i32 %i) {
 ; CHECK-LABEL: @i32_cast_cmp_slt_int_0_uitofp_float(
-; CHECK-NEXT:    [[F:%.*]] = uitofp i32 [[I:%.*]] to float
-; CHECK-NEXT:    [[B:%.*]] = bitcast float [[F]] to i32
-; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[B]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 false
 ;
   %f = uitofp i32 %i to float
   %b = bitcast float %f to i32
@@ -25,10 +27,7 @@
 
 define <2 x i1> @i32_cast_cmp_slt_int_0_uitofp_float_vec(<2 x i32> %i) {
 ; CHECK-LABEL: @i32_cast_cmp_slt_int_0_uitofp_float_vec(
-; CHECK-NEXT:    [[F:%.*]] = uitofp <2 x i32> [[I:%.*]] to <2 x float>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <2 x float> [[F]] to <2 x i32>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp slt <2 x i32> [[B]], zeroinitializer
-; CHECK-NEXT:    ret <2 x i1> [[CMP]]
+; CHECK-NEXT:    ret <2 x i1> zeroinitializer
 ;
   %f = uitofp <2 x i32> %i to <2 x float>
   %b = bitcast <2 x float> %f to <2 x i32>
@@ -51,10 +50,7 @@
 
 define i1 @i32_cast_cmp_sgt_int_m1_uitofp_float(i32 %i) {
 ; CHECK-LABEL: @i32_cast_cmp_sgt_int_m1_uitofp_float(
-; CHECK-NEXT:    [[F:%.*]] = uitofp i32 [[I:%.*]] to float
-; CHECK-NEXT:    [[B:%.*]] = bitcast float [[F]] to i32
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i32 [[B]], -1
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 true
 ;
   %f = uitofp i32 %i to float
   %b = bitcast float %f to i32
@@ -64,10 +60,7 @@
 
 define <2 x i1> @i32_cast_cmp_sgt_int_m1_uitofp_float_vec(<2 x i32> %i) {
 ; CHECK-LABEL: @i32_cast_cmp_sgt_int_m1_uitofp_float_vec(
-; CHECK-NEXT:    [[F:%.*]] = uitofp <2 x i32> [[I:%.*]] to <2 x float>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <2 x float> [[F]] to <2 x i32>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <2 x i32> [[B]], <i32 -1, i32 -1>
-; CHECK-NEXT:    ret <2 x i1> [[CMP]]
+; CHECK-NEXT:    ret <2 x i1> <i1 true, i1 true>
 ;
   %f = uitofp <2 x i32> %i to <2 x float>
   %b = bitcast <2 x float> %f to <2 x i32>
@@ -77,10 +70,7 @@
 
 define <3 x i1> @i32_cast_cmp_sgt_int_m1_uitofp_float_vec_undef(<3 x i32> %i) {
 ; CHECK-LABEL: @i32_cast_cmp_sgt_int_m1_uitofp_float_vec_undef(
-; CHECK-NEXT:    [[F:%.*]] = uitofp <3 x i32> [[I:%.*]] to <3 x float>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <3 x float> [[F]] to <3 x i32>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <3 x i32> [[B]], <i32 -1, i32 undef, i32 -1>
-; CHECK-NEXT:    ret <3 x i1> [[CMP]]
+; CHECK-NEXT:    ret <3 x i1> <i1 true, i1 true, i1 true>
 ;
   %f = uitofp <3 x i32> %i to <3 x float>
   %b = bitcast <3 x float> %f to <3 x i32>
@@ -90,10 +80,7 @@
 
 define i1 @i32_cast_cmp_slt_int_0_uitofp_double(i32 %i) {
 ; CHECK-LABEL: @i32_cast_cmp_slt_int_0_uitofp_double(
-; CHECK-NEXT:    [[F:%.*]] = uitofp i32 [[I:%.*]] to double
-; CHECK-NEXT:    [[B:%.*]] = bitcast double [[F]] to i64
-; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i64 [[B]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 false
 ;
   %f = uitofp i32 %i to double
   %b = bitcast double %f to i64
@@ -103,10 +90,7 @@
 
 define <2 x i1> @i32_cast_cmp_slt_int_0_uitofp_double_vec(<2 x i32> %i) {
 ; CHECK-LABEL: @i32_cast_cmp_slt_int_0_uitofp_double_vec(
-; CHECK-NEXT:    [[F:%.*]] = uitofp <2 x i32> [[I:%.*]] to <2 x double>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <2 x double> [[F]] to <2 x i64>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp slt <2 x i64> [[B]], zeroinitializer
-; CHECK-NEXT:    ret <2 x i1> [[CMP]]
+; CHECK-NEXT:    ret <2 x i1> zeroinitializer
 ;
   %f = uitofp <2 x i32> %i to <2 x double>
   %b = bitcast <2 x double> %f to <2 x i64>
@@ -129,10 +113,7 @@
 
 define i1 @i32_cast_cmp_sgt_int_m1_uitofp_double(i32 %i) {
 ; CHECK-LABEL: @i32_cast_cmp_sgt_int_m1_uitofp_double(
-; CHECK-NEXT:    [[F:%.*]] = uitofp i32 [[I:%.*]] to double
-; CHECK-NEXT:    [[B:%.*]] = bitcast double [[F]] to i64
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i64 [[B]], -1
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 true
 ;
   %f = uitofp i32 %i to double
   %b = bitcast double %f to i64
@@ -142,10 +123,7 @@
 
 define <2 x i1> @i32_cast_cmp_sgt_int_m1_uitofp_double_vec(<2 x i32> %i) {
 ; CHECK-LABEL: @i32_cast_cmp_sgt_int_m1_uitofp_double_vec(
-; CHECK-NEXT:    [[F:%.*]] = uitofp <2 x i32> [[I:%.*]] to <2 x double>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <2 x double> [[F]] to <2 x i64>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <2 x i64> [[B]], <i64 -1, i64 -1>
-; CHECK-NEXT:    ret <2 x i1> [[CMP]]
+; CHECK-NEXT:    ret <2 x i1> <i1 true, i1 true>
 ;
   %f = uitofp <2 x i32> %i to <2 x double>
   %b = bitcast <2 x double> %f to <2 x i64>
@@ -155,10 +133,7 @@
 
 define <3 x i1> @i32_cast_cmp_sgt_int_m1_uitofp_double_vec_undef(<3 x i32> %i) {
 ; CHECK-LABEL: @i32_cast_cmp_sgt_int_m1_uitofp_double_vec_undef(
-; CHECK-NEXT:    [[F:%.*]] = uitofp <3 x i32> [[I:%.*]] to <3 x double>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <3 x double> [[F]] to <3 x i64>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <3 x i64> [[B]], <i64 -1, i64 undef, i64 -1>
-; CHECK-NEXT:    ret <3 x i1> [[CMP]]
+; CHECK-NEXT:    ret <3 x i1> <i1 true, i1 true, i1 true>
 ;
   %f = uitofp <3 x i32> %i to <3 x double>
   %b = bitcast <3 x double> %f to <3 x i64>
@@ -168,10 +143,7 @@
 
 define i1 @i32_cast_cmp_slt_int_0_uitofp_half(i32 %i) {
 ; CHECK-LABEL: @i32_cast_cmp_slt_int_0_uitofp_half(
-; CHECK-NEXT:    [[F:%.*]] = uitofp i32 [[I:%.*]] to half
-; CHECK-NEXT:    [[B:%.*]] = bitcast half [[F]] to i16
-; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i16 [[B]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 false
 ;
   %f = uitofp i32 %i to half
   %b = bitcast half %f to i16
@@ -181,10 +153,7 @@
 
 define <2 x i1> @i32_cast_cmp_slt_int_0_uitofp_half_vec(<2 x i32> %i) {
 ; CHECK-LABEL: @i32_cast_cmp_slt_int_0_uitofp_half_vec(
-; CHECK-NEXT:    [[F:%.*]] = uitofp <2 x i32> [[I:%.*]] to <2 x half>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <2 x half> [[F]] to <2 x i16>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp slt <2 x i16> [[B]], zeroinitializer
-; CHECK-NEXT:    ret <2 x i1> [[CMP]]
+; CHECK-NEXT:    ret <2 x i1> zeroinitializer
 ;
   %f = uitofp <2 x i32> %i to <2 x half>
   %b = bitcast <2 x half> %f to <2 x i16>
@@ -207,10 +176,7 @@
 
 define i1 @i32_cast_cmp_sgt_int_m1_uitofp_half(i32 %i) {
 ; CHECK-LABEL: @i32_cast_cmp_sgt_int_m1_uitofp_half(
-; CHECK-NEXT:    [[F:%.*]] = uitofp i32 [[I:%.*]] to half
-; CHECK-NEXT:    [[B:%.*]] = bitcast half [[F]] to i16
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i16 [[B]], -1
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 true
 ;
   %f = uitofp i32 %i to half
   %b = bitcast half %f to i16
@@ -220,10 +186,7 @@
 
 define <2 x i1> @i32_cast_cmp_sgt_int_m1_uitofp_half_vec(<2 x i32> %i) {
 ; CHECK-LABEL: @i32_cast_cmp_sgt_int_m1_uitofp_half_vec(
-; CHECK-NEXT:    [[F:%.*]] = uitofp <2 x i32> [[I:%.*]] to <2 x half>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <2 x half> [[F]] to <2 x i16>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <2 x i16> [[B]], <i16 -1, i16 -1>
-; CHECK-NEXT:    ret <2 x i1> [[CMP]]
+; CHECK-NEXT:    ret <2 x i1> <i1 true, i1 true>
 ;
   %f = uitofp <2 x i32> %i to <2 x half>
   %b = bitcast <2 x half> %f to <2 x i16>
@@ -233,10 +196,7 @@
 
 define <3 x i1> @i32_cast_cmp_sgt_int_m1_uitofp_half_vec_undef(<3 x i32> %i) {
 ; CHECK-LABEL: @i32_cast_cmp_sgt_int_m1_uitofp_half_vec_undef(
-; CHECK-NEXT:    [[F:%.*]] = uitofp <3 x i32> [[I:%.*]] to <3 x half>
-; CHECK-NEXT:    [[B:%.*]] = bitcast <3 x half> [[F]] to <3 x i16>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <3 x i16> [[B]], <i16 -1, i16 undef, i16 -1>
-; CHECK-NEXT:    ret <3 x i1> [[CMP]]
+; CHECK-NEXT:    ret <3 x i1> <i1 true, i1 true, i1 true>
 ;
   %f = uitofp <3 x i32> %i to <3 x half>
   %b = bitcast <3 x half> %f to <3 x i16>