[llvm] r315152 - [InstSimplify] add tests to show we can do better at folding poison; NFC
Sanjay Patel via llvm-commits
llvm-commits at lists.llvm.org
Sat Oct 7 08:39:06 PDT 2017
Author: spatel
Date: Sat Oct 7 08:39:06 2017
New Revision: 315152
URL: http://llvm.org/viewvc/llvm-project?rev=315152&view=rev
Log:
[InstSimplify] add tests to show we can do better at folding poison; NFC
Modified:
llvm/trunk/test/Transforms/InstSimplify/icmp-constant.ll
Modified: llvm/trunk/test/Transforms/InstSimplify/icmp-constant.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/icmp-constant.ll?rev=315152&r1=315151&r2=315152&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/icmp-constant.ll (original)
+++ llvm/trunk/test/Transforms/InstSimplify/icmp-constant.ll Sat Oct 7 08:39:06 2017
@@ -571,3 +571,61 @@ define <2 x i1> @add_nsw_pos_const5_spla
ret <2 x i1> %cmp
}
+; PR34838 - https://bugs.llvm.org/show_bug.cgi?id=34838
+; The shift is known to create poison, so we can simplify the cmp.
+
+define i1 @ne_shl_by_constant_produces_poison(i8 %x) {
+; CHECK-LABEL: @ne_shl_by_constant_produces_poison(
+; CHECK-NEXT: [[ZX:%.*]] = zext i8 %x to i16
+; CHECK-NEXT: [[XOR:%.*]] = xor i16 [[ZX]], 32767
+; CHECK-NEXT: [[SUB:%.*]] = sub nsw i16 [[ZX]], [[XOR]]
+; CHECK-NEXT: [[POISON:%.*]] = shl nsw i16 [[SUB]], 2
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i16 [[POISON]], 1
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %zx = zext i8 %x to i16 ; zx = 0x00xx
+ %xor = xor i16 %zx, 32767 ; xor = 0x7fyy
+ %sub = sub nsw i16 %zx, %xor ; sub = 0x80zz (the top bit is known one)
+ %poison = shl nsw i16 %sub, 2 ; oops! this shl can't be nsw; that's POISON
+ %cmp = icmp ne i16 %poison, 1
+ ret i1 %cmp
+}
+
+define i1 @eq_shl_by_constant_produces_poison(i8 %x) {
+; CHECK-LABEL: @eq_shl_by_constant_produces_poison(
+; CHECK-NEXT: [[CLEAR_HIGH_BIT:%.*]] = and i8 %x, 127
+; CHECK-NEXT: [[SET_NEXT_HIGH_BITS:%.*]] = or i8 [[CLEAR_HIGH_BIT]], 112
+; CHECK-NEXT: [[POISON:%.*]] = shl nsw i8 [[SET_NEXT_HIGH_BITS]], 3
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i8 [[POISON]], 15
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %clear_high_bit = and i8 %x, 127 ; 0x7f
+ %set_next_high_bits = or i8 %clear_high_bit, 112 ; 0x70
+ %poison = shl nsw i8 %set_next_high_bits, 3
+ %cmp = icmp eq i8 %poison, 15
+ ret i1 %cmp
+}
+
+; Shift-by-variable that produces poison is more complicated but still possible.
+; We guarantee that the shift will change the sign of the shifted value (and
+; therefore produce poison) by limiting its range from 1 to 3.
+
+define i1 @eq_shl_by_variable_produces_poison(i8 %x) {
+; CHECK-LABEL: @eq_shl_by_variable_produces_poison(
+; CHECK-NEXT: [[CLEAR_HIGH_BIT:%.*]] = and i8 %x, 127
+; CHECK-NEXT: [[SET_NEXT_HIGH_BITS:%.*]] = or i8 [[CLEAR_HIGH_BIT]], 112
+; CHECK-NEXT: [[NOTUNDEF_SHIFTAMT:%.*]] = and i8 %x, 3
+; CHECK-NEXT: [[NONZERO_SHIFTAMT:%.*]] = or i8 [[NOTUNDEF_SHIFTAMT]], 1
+; CHECK-NEXT: [[POISON:%.*]] = shl nsw i8 [[SET_NEXT_HIGH_BITS]], [[NONZERO_SHIFTAMT]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i8 [[POISON]], 15
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %clear_high_bit = and i8 %x, 127 ; 0x7f
+ %set_next_high_bits = or i8 %clear_high_bit, 112 ; 0x70
+ %notundef_shiftamt = and i8 %x, 3
+ %nonzero_shiftamt = or i8 %notundef_shiftamt, 1
+ %poison = shl nsw i8 %set_next_high_bits, %nonzero_shiftamt
+ %cmp = icmp eq i8 %poison, 15
+ ret i1 %cmp
+}
+
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