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[InstCombine] add nsw/nuw X, signbit --> or X, signbit 

Changing to 'or' (rather than 'xor' when no wrapping flags are set)
allows icmp simplifies to happen as expected.

Differential Revision: https://reviews.llvm.org/D29729

llvm-svn: 295574
This commit is contained in:
Sanjay Patel 2017-02-18 22:20:09 +00:00
parent fe67255961
commit 53c5c3d65d
3 changed files with 17 additions and 14 deletions
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11
llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
View File

@ -1044,9 +1044,16 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
const APInt *Val;
if (match(RHS, m_APInt(Val))) {
// X + (signbit) --> X ^ signbit
if (Val->isSignBit())
if (Val->isSignBit()) {
// If wrapping is not allowed, then the addition must set the sign bit:
// X + (signbit) --> X | signbit
if (I.hasNoSignedWrap() || I.hasNoUnsignedWrap())
return BinaryOperator::CreateOr(LHS, RHS);
// If wrapping is allowed, then the addition flips the sign bit of LHS:
// X + (signbit) --> X ^ signbit
return BinaryOperator::CreateXor(LHS, RHS);
}
// Is this add the last step in a convoluted sext?
Value *X;

10
llvm/test/Transforms/InstCombine/add.ll
View File

@ -267,24 +267,22 @@ define i32 @xor_sign_bit(i32 %x) {
ret i32 %add
}
; Lose no-wrap info by converting to xor? %x is known non-negative
; here, but not after converting to xor.
; No-wrap info allows converting the add to 'or'.
define i8 @add_nsw_signbit(i8 %x) {
; CHECK-LABEL: @add_nsw_signbit(
; CHECK-NEXT: [[Y:%.*]] = xor i8 %x, -128
; CHECK-NEXT: [[Y:%.*]] = or i8 %x, -128
; CHECK-NEXT: ret i8 [[Y]]
;
%y = add nsw i8 %x, -128
ret i8 %y
}
; Lose no-wrap info by converting to xor? %x is known non-negative
; (x < 128 unsigned), but not after converting to xor.
; No-wrap info allows converting the add to 'or'.
define i8 @add_nuw_signbit(i8 %x) {
; CHECK-LABEL: @add_nuw_signbit(
; CHECK-NEXT: [[Y:%.*]] = xor i8 %x, -128
; CHECK-NEXT: [[Y:%.*]] = or i8 %x, -128
; CHECK-NEXT: ret i8 [[Y]]
;
%y = add nuw i8 %x, 128

10
llvm/test/Transforms/InstCombine/icmp-add.ll
View File

@ -223,24 +223,22 @@ define i1 @nsw_slt5_ov(i8 %a) {
ret i1 %c
}
; FIXME: InstCombine should not lose wrapping information by changing the add to xor.
; InstCombine should not thwart this opportunity to simplify completely.
define i1 @slt_zero_add_nsw_signbit(i8 %x) {
; CHECK-LABEL: @slt_zero_add_nsw_signbit(
; CHECK-NEXT: [[Z:%.*]] = icmp sgt i8 %x, -1
; CHECK-NEXT: ret i1 [[Z]]
; CHECK-NEXT: ret i1 true
;
%y = add nsw i8 %x, -128
%z = icmp slt i8 %y, 0
ret i1 %z
}
; FIXME: InstCombine should not lose wrapping information by changing the add to xor.
; InstCombine should not thwart this opportunity to simplify completely.
define i1 @slt_zero_add_nuw_signbit(i8 %x) {
; CHECK-LABEL: @slt_zero_add_nuw_signbit(
; CHECK-NEXT: [[Z:%.*]] = icmp sgt i8 %x, -1
; CHECK-NEXT: ret i1 [[Z]]
; CHECK-NEXT: ret i1 true
;
%y = add nuw i8 %x, 128
%z = icmp slt i8 %y, 0