[NFC][InstCombine] PR38708 - inefficient pattern for high-bits checking 3.

The last, non-canonical variant:
https://godbolt.org/z/aCMsPk
https://rise4fun.com/Alive/I6f

It can only happen due to the extra use on the inner shift.
But here it is ok.

https://bugs.llvm.org/show_bug.cgi?id=38708

llvm-svn: 342184

2 files changed, 520 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/icmp-uge-of-add-of-shl-one-by-bits-to-allones-and-val-to-icmp-eq-of-lshr-val-by-bits-and-0.ll b/llvm/test/Transforms/InstCombine/icmp-uge-of-add-of-shl-one-by-bits-to-allones-and-val-to-icmp-eq-of-lshr-val-by-bits-and-0.ll
new file mode 100644
index 00000000000..d3256aade6a
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/icmp-uge-of-add-of-shl-one-by-bits-to-allones-and-val-to-icmp-eq-of-lshr-val-by-bits-and-0.ll
@@ -0,0 +1,260 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+; https://bugs.llvm.org/show_bug.cgi?id=38708
+
+; Pattern:
+;   ((1 << bits)+(-1)) u>= val
+; Should be transformed into:
+;   (val l>> bits) == 0
+
+; NOTE: the innermost shl is not one-use. Else canonicalization happens.
+
+declare void @use8(i8)
+declare void @use2i8(<2 x i8>)
+declare void @use3i8(<3 x i8>)
+
+; ============================================================================ ;
+; Basic positive tests
+; ============================================================================ ;
+
+define i1 @p0(i8 %val, i8 %bits) {
+; CHECK-LABEL: @p0(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[R:%.*]] = icmp uge i8 [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %r = icmp uge i8 %t1, %val
+  ret i1 %r
+}
+
+; ============================================================================ ;
+; Vector tests
+; ============================================================================ ;
+
+define <2 x i1> @p1_vec(<2 x i8> %val, <2 x i8> %bits) {
+; CHECK-LABEL: @p1_vec(
+; CHECK-NEXT:    [[T0:%.*]] = shl <2 x i8> <i8 1, i8 1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use2i8(<2 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <2 x i8> [[T0]], <i8 -1, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp uge <2 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <2 x i1> [[R]]
+;
+  %t0 = shl <2 x i8> <i8 1, i8 1>, %bits
+  call void @use2i8(<2 x i8> %t0)
+  %t1 = add <2 x i8> %t0, <i8 -1, i8 -1>
+  %r = icmp uge <2 x i8> %t1, %val
+  ret <2 x i1> %r
+}
+
+define <3 x i1> @p2_vec_undef0(<3 x i8> %val, <3 x i8> %bits) {
+; CHECK-LABEL: @p2_vec_undef0(
+; CHECK-NEXT:    [[T0:%.*]] = shl <3 x i8> <i8 1, i8 undef, i8 1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use3i8(<3 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <3 x i8> [[T0]], <i8 -1, i8 -1, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp uge <3 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <3 x i1> [[R]]
+;
+  %t0 = shl <3 x i8> <i8 1, i8 undef, i8 1>, %bits
+  call void @use3i8(<3 x i8> %t0)
+  %t1 = add <3 x i8> %t0, <i8 -1, i8 -1, i8 -1>
+  %r = icmp uge <3 x i8> %t1, %val
+  ret <3 x i1> %r
+}
+
+define <3 x i1> @p2_vec_undef1(<3 x i8> %val, <3 x i8> %bits) {
+; CHECK-LABEL: @p2_vec_undef1(
+; CHECK-NEXT:    [[T0:%.*]] = shl <3 x i8> <i8 1, i8 1, i8 1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use3i8(<3 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <3 x i8> [[T0]], <i8 -1, i8 undef, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp uge <3 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <3 x i1> [[R]]
+;
+  %t0 = shl <3 x i8> <i8 1, i8 1, i8 1>, %bits
+  call void @use3i8(<3 x i8> %t0)
+  %t1 = add <3 x i8> %t0, <i8 -1, i8 undef, i8 -1>
+  %r = icmp uge <3 x i8> %t1, %val
+  ret <3 x i1> %r
+}
+
+define <3 x i1> @p2_vec_undef2(<3 x i8> %val, <3 x i8> %bits) {
+; CHECK-LABEL: @p2_vec_undef2(
+; CHECK-NEXT:    [[T0:%.*]] = shl <3 x i8> <i8 1, i8 undef, i8 1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use3i8(<3 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <3 x i8> [[T0]], <i8 -1, i8 undef, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp uge <3 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <3 x i1> [[R]]
+;
+  %t0 = shl <3 x i8> <i8 1, i8 undef, i8 1>, %bits
+  call void @use3i8(<3 x i8> %t0)
+  %t1 = add <3 x i8> %t0, <i8 -1, i8 undef, i8 -1>
+  %r = icmp uge <3 x i8> %t1, %val
+  ret <3 x i1> %r
+}
+
+; ============================================================================ ;
+; Commutativity tests.
+; ============================================================================ ;
+
+declare i8 @gen8()
+
+define i1 @c0(i8 %bits) {
+; CHECK-LABEL: @c0(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[VAL:%.*]] = call i8 @gen8()
+; CHECK-NEXT:    [[R:%.*]] = icmp ule i8 [[VAL]], [[T1]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %val = call i8 @gen8()
+  %r = icmp ule i8 %val, %t1 ; swapped order and predicate
+  ret i1 %r
+}
+
+; What if we have the same pattern on both sides?
+define i1 @both(i8 %bits0, i8 %bits1) {
+; CHECK-LABEL: @both(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS0:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[T2:%.*]] = shl i8 1, [[BITS1:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T2]])
+; CHECK-NEXT:    [[T3:%.*]] = add i8 [[T2]], -1
+; CHECK-NEXT:    [[R:%.*]] = icmp uge i8 [[T1]], [[T3]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits0
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %t2 = shl i8 1, %bits1
+  call void @use8(i8 %t2)
+  %t3 = add i8 %t2, -1
+  %r = icmp uge i8 %t1, %t3
+  ret i1 %r
+}
+
+; ============================================================================ ;
+; One-use tests.
+; ============================================================================ ;
+
+define i1 @oneuse(i8 %val, i8 %bits) {
+; CHECK-LABEL: @oneuse(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    call void @use8(i8 [[T1]])
+; CHECK-NEXT:    [[R:%.*]] = icmp uge i8 [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0) ; this is needed anyway
+  %t1 = add i8 %t0, -1
+  call void @use8(i8 %t1)
+  %r = icmp uge i8 %t1, %val
+  ret i1 %r
+}
+
+; ============================================================================ ;
+; Negative tests
+; ============================================================================ ;
+
+define i1 @n0(i8 %val, i8 %bits) {
+; CHECK-LABEL: @n0(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 -1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[R:%.*]] = icmp uge i8 [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 -1, %bits ; constant is not 1
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %r = icmp uge i8 %t1, %val
+  ret i1 %r
+}
+
+define i1 @n1(i8 %val, i8 %bits) {
+; CHECK-LABEL: @n1(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], 1
+; CHECK-NEXT:    [[R:%.*]] = icmp uge i8 [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, 1 ; constant is not -1
+  %r = icmp uge i8 %t1, %val
+  ret i1 %r
+}
+
+define <2 x i1> @n2_vec_nonsplat(<2 x i8> %val, <2 x i8> %bits) {
+; CHECK-LABEL: @n2_vec_nonsplat(
+; CHECK-NEXT:    [[T0:%.*]] = shl <2 x i8> <i8 1, i8 -1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use2i8(<2 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <2 x i8> [[T0]], <i8 -1, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp uge <2 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <2 x i1> [[R]]
+;
+  %t0 = shl <2 x i8> <i8 1, i8 -1>, %bits ; again, wrong constant
+  call void @use2i8(<2 x i8> %t0)
+  %t1 = add <2 x i8> %t0, <i8 -1, i8 -1>
+  %r = icmp uge <2 x i8> %t1, %val
+  ret <2 x i1> %r
+}
+
+define <2 x i1> @n3_vec_nonsplat(<2 x i8> %val, <2 x i8> %bits) {
+; CHECK-LABEL: @n3_vec_nonsplat(
+; CHECK-NEXT:    [[T0:%.*]] = shl <2 x i8> <i8 1, i8 1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use2i8(<2 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <2 x i8> [[T0]], <i8 -1, i8 1>
+; CHECK-NEXT:    [[R:%.*]] = icmp uge <2 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <2 x i1> [[R]]
+;
+  %t0 = shl <2 x i8> <i8 1, i8 1>, %bits
+  call void @use2i8(<2 x i8> %t0)
+  %t1 = add <2 x i8> %t0, <i8 -1, i8 1> ; again, wrong constant
+  %r = icmp uge <2 x i8> %t1, %val
+  ret <2 x i1> %r
+}
+
+define i1 @n3(i8 %val, i8 %bits) {
+; CHECK-LABEL: @n3(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[R:%.*]] = icmp ugt i8 [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %r = icmp ugt i8 %t1, %val ; wrong predicate
+  ret i1 %r
+}
+
+define i1 @n4(i8 %bits) {
+; CHECK-LABEL: @n4(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[VAL:%.*]] = call i8 @gen8()
+; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[VAL]], [[T1]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %val = call i8 @gen8()
+  %r = icmp ult i8 %val, %t1 ; swapped order and [wrong] predicate
+  ret i1 %r
+}
diff --git a/llvm/test/Transforms/InstCombine/icmp-ult-of-add-of-shl-one-by-bits-to-allones-and-val-to-icmp-ne-of-lshr-val-by-bits-and-0.ll b/llvm/test/Transforms/InstCombine/icmp-ult-of-add-of-shl-one-by-bits-to-allones-and-val-to-icmp-ne-of-lshr-val-by-bits-and-0.ll
new file mode 100644
index 00000000000..d68dd711835
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/icmp-ult-of-add-of-shl-one-by-bits-to-allones-and-val-to-icmp-ne-of-lshr-val-by-bits-and-0.ll
@@ -0,0 +1,260 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+; https://bugs.llvm.org/show_bug.cgi?id=38708
+
+; Pattern:
+;   ((1 << bits)+(-1)) u< val
+; Should be transformed into:
+;   (val l>> bits) != 0
+
+; NOTE: the innermost shl is not one-use. Else canonicalization happens.
+
+declare void @use8(i8)
+declare void @use2i8(<2 x i8>)
+declare void @use3i8(<3 x i8>)
+
+; ============================================================================ ;
+; Basic positive tests
+; ============================================================================ ;
+
+define i1 @p0(i8 %val, i8 %bits) {
+; CHECK-LABEL: @p0(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %r = icmp ult i8 %t1, %val
+  ret i1 %r
+}
+
+; ============================================================================ ;
+; Vector tests
+; ============================================================================ ;
+
+define <2 x i1> @p1_vec(<2 x i8> %val, <2 x i8> %bits) {
+; CHECK-LABEL: @p1_vec(
+; CHECK-NEXT:    [[T0:%.*]] = shl <2 x i8> <i8 1, i8 1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use2i8(<2 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <2 x i8> [[T0]], <i8 -1, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp ult <2 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <2 x i1> [[R]]
+;
+  %t0 = shl <2 x i8> <i8 1, i8 1>, %bits
+  call void @use2i8(<2 x i8> %t0)
+  %t1 = add <2 x i8> %t0, <i8 -1, i8 -1>
+  %r = icmp ult <2 x i8> %t1, %val
+  ret <2 x i1> %r
+}
+
+define <3 x i1> @p2_vec_undef0(<3 x i8> %val, <3 x i8> %bits) {
+; CHECK-LABEL: @p2_vec_undef0(
+; CHECK-NEXT:    [[T0:%.*]] = shl <3 x i8> <i8 1, i8 undef, i8 1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use3i8(<3 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <3 x i8> [[T0]], <i8 -1, i8 -1, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp ult <3 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <3 x i1> [[R]]
+;
+  %t0 = shl <3 x i8> <i8 1, i8 undef, i8 1>, %bits
+  call void @use3i8(<3 x i8> %t0)
+  %t1 = add <3 x i8> %t0, <i8 -1, i8 -1, i8 -1>
+  %r = icmp ult <3 x i8> %t1, %val
+  ret <3 x i1> %r
+}
+
+define <3 x i1> @p2_vec_undef1(<3 x i8> %val, <3 x i8> %bits) {
+; CHECK-LABEL: @p2_vec_undef1(
+; CHECK-NEXT:    [[T0:%.*]] = shl <3 x i8> <i8 1, i8 1, i8 1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use3i8(<3 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <3 x i8> [[T0]], <i8 -1, i8 undef, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp ult <3 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <3 x i1> [[R]]
+;
+  %t0 = shl <3 x i8> <i8 1, i8 1, i8 1>, %bits
+  call void @use3i8(<3 x i8> %t0)
+  %t1 = add <3 x i8> %t0, <i8 -1, i8 undef, i8 -1>
+  %r = icmp ult <3 x i8> %t1, %val
+  ret <3 x i1> %r
+}
+
+define <3 x i1> @p2_vec_undef2(<3 x i8> %val, <3 x i8> %bits) {
+; CHECK-LABEL: @p2_vec_undef2(
+; CHECK-NEXT:    [[T0:%.*]] = shl <3 x i8> <i8 1, i8 undef, i8 1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use3i8(<3 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <3 x i8> [[T0]], <i8 -1, i8 undef, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp ult <3 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <3 x i1> [[R]]
+;
+  %t0 = shl <3 x i8> <i8 1, i8 undef, i8 1>, %bits
+  call void @use3i8(<3 x i8> %t0)
+  %t1 = add <3 x i8> %t0, <i8 -1, i8 undef, i8 -1>
+  %r = icmp ult <3 x i8> %t1, %val
+  ret <3 x i1> %r
+}
+
+; ============================================================================ ;
+; Commutativity tests.
+; ============================================================================ ;
+
+declare i8 @gen8()
+
+define i1 @c0(i8 %bits) {
+; CHECK-LABEL: @c0(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[VAL:%.*]] = call i8 @gen8()
+; CHECK-NEXT:    [[R:%.*]] = icmp ugt i8 [[VAL]], [[T1]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %val = call i8 @gen8()
+  %r = icmp ugt i8 %val, %t1 ; swapped order and predicate
+  ret i1 %r
+}
+
+; What if we have the same pattern on both sides?
+define i1 @both(i8 %bits0, i8 %bits1) {
+; CHECK-LABEL: @both(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS0:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[T2:%.*]] = shl i8 1, [[BITS1:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T2]])
+; CHECK-NEXT:    [[T3:%.*]] = add i8 [[T2]], -1
+; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[T1]], [[T3]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits0
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %t2 = shl i8 1, %bits1
+  call void @use8(i8 %t2)
+  %t3 = add i8 %t2, -1
+  %r = icmp ult i8 %t1, %t3
+  ret i1 %r
+}
+
+; ============================================================================ ;
+; One-use tests.
+; ============================================================================ ;
+
+define i1 @oneuse(i8 %val, i8 %bits) {
+; CHECK-LABEL: @oneuse(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    call void @use8(i8 [[T1]])
+; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0) ; this is needed anyway
+  %t1 = add i8 %t0, -1
+  call void @use8(i8 %t1)
+  %r = icmp ult i8 %t1, %val
+  ret i1 %r
+}
+
+; ============================================================================ ;
+; Negative tests
+; ============================================================================ ;
+
+define i1 @n0(i8 %val, i8 %bits) {
+; CHECK-LABEL: @n0(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 -1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 -1, %bits ; constant is not 1
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %r = icmp ult i8 %t1, %val
+  ret i1 %r
+}
+
+define i1 @n1(i8 %val, i8 %bits) {
+; CHECK-LABEL: @n1(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], 1
+; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, 1 ; constant is not -1
+  %r = icmp ult i8 %t1, %val
+  ret i1 %r
+}
+
+define <2 x i1> @n2_vec_nonsplat(<2 x i8> %val, <2 x i8> %bits) {
+; CHECK-LABEL: @n2_vec_nonsplat(
+; CHECK-NEXT:    [[T0:%.*]] = shl <2 x i8> <i8 1, i8 -1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use2i8(<2 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <2 x i8> [[T0]], <i8 -1, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp ult <2 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <2 x i1> [[R]]
+;
+  %t0 = shl <2 x i8> <i8 1, i8 -1>, %bits ; again, wrong constant
+  call void @use2i8(<2 x i8> %t0)
+  %t1 = add <2 x i8> %t0, <i8 -1, i8 -1>
+  %r = icmp ult <2 x i8> %t1, %val
+  ret <2 x i1> %r
+}
+
+define <2 x i1> @n3_vec_nonsplat(<2 x i8> %val, <2 x i8> %bits) {
+; CHECK-LABEL: @n3_vec_nonsplat(
+; CHECK-NEXT:    [[T0:%.*]] = shl <2 x i8> <i8 1, i8 1>, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use2i8(<2 x i8> [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add <2 x i8> [[T0]], <i8 -1, i8 1>
+; CHECK-NEXT:    [[R:%.*]] = icmp ult <2 x i8> [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret <2 x i1> [[R]]
+;
+  %t0 = shl <2 x i8> <i8 1, i8 1>, %bits
+  call void @use2i8(<2 x i8> %t0)
+  %t1 = add <2 x i8> %t0, <i8 -1, i8 1> ; again, wrong constant
+  %r = icmp ult <2 x i8> %t1, %val
+  ret <2 x i1> %r
+}
+
+define i1 @n3(i8 %val, i8 %bits) {
+; CHECK-LABEL: @n3(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[R:%.*]] = icmp ule i8 [[T1]], [[VAL:%.*]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %r = icmp ule i8 %t1, %val ; wrong predicate
+  ret i1 %r
+}
+
+define i1 @n4(i8 %bits) {
+; CHECK-LABEL: @n4(
+; CHECK-NEXT:    [[T0:%.*]] = shl i8 1, [[BITS:%.*]]
+; CHECK-NEXT:    call void @use8(i8 [[T0]])
+; CHECK-NEXT:    [[T1:%.*]] = add i8 [[T0]], -1
+; CHECK-NEXT:    [[VAL:%.*]] = call i8 @gen8()
+; CHECK-NEXT:    [[R:%.*]] = icmp uge i8 [[VAL]], [[T1]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %t0 = shl i8 1, %bits
+  call void @use8(i8 %t0)
+  %t1 = add i8 %t0, -1
+  %val = call i8 @gen8()
+  %r = icmp uge i8 %val, %t1 ; swapped order and [wrong] predicate
+  ret i1 %r
+}