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[ValueTracking] Teach computeKnownBits that the result of an absolute value pattern that uses nsw flag is always positive.

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If the nsw flag is used in the absolute value then it is undefined for INT_MIN. For all other value it will produce a positive number. So we can assume the result is positive.

This breaks some InstCombine abs/nabs combining tests because we simplify the second compare from known bits rather than as the whole pattern. Looks like we can probably fix it by adding a neg+abs/nabs combine to just swap the select operands. Need to check alive to make sure there are no corner cases.

Differential Revision: https://reviews.llvm.org/D47041

llvm-svn: 333226
This commit is contained in:
Craig Topper 2018-05-24 21:22:51 +00:00
parent 74ca133d49
commit 77f076a24d
3 changed files with 25 additions and 8 deletions
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6
lib/Analysis/ValueTracking.cpp
View File

@ -1078,6 +1078,12 @@ static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
// leading zero bits.
MaxHighZeros =
std::max(Known.countMinLeadingZeros(), Known2.countMinLeadingZeros());
} else if (SPF == SPF_ABS) {
// RHS from matchSelectPattern returns the negation part of abs pattern.
// If the negate has an NSW flag we can assume the sign bit of the result
// will be 0 because that makes abs(INT_MIN) undefined.
if (cast<Instruction>(RHS)->hasNoSignedWrap())
MaxHighZeros = 1;
}
// Only known if known in both the LHS and RHS.

11
test/Transforms/InstCombine/abs-1.ll
View File

@ -293,3 +293,14 @@ define <2 x i8> @negate_nabs(<2 x i8> %x) {
%r = sub <2 x i8> zeroinitializer, %s
ret <2 x i8> %r
}
define i1 @abs_must_be_positive(i32 %x) {
; CHECK-LABEL: @abs_must_be_positive(
; CHECK-NEXT: ret i1 true
;
%negx = sub nsw i32 0, %x
%c = icmp sge i32 %x, 0
%sel = select i1 %c, i32 %x, i32 %negx
%c2 = icmp sge i32 %sel, 0
ret i1 %c2
}

16
test/Transforms/InstCombine/abs_abs.ll
View File

@ -933,8 +933,8 @@ define i32 @nabs_abs_x09(i32 %x) {
; CHECK-LABEL: @nabs_abs_x09(
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
; CHECK-NEXT: [[SUB:%.*]] = sub nsw i32 0, [[X]]
; CHECK-NEXT: [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
; CHECK-NEXT: ret i32 [[COND1]]
; CHECK-NEXT: [[COND:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
; CHECK-NEXT: ret i32 [[COND]]
;
%cmp = icmp sgt i32 %x, -1
%sub = sub nsw i32 0, %x
@ -949,8 +949,8 @@ define i32 @nabs_abs_x10(i32 %x) {
; CHECK-LABEL: @nabs_abs_x10(
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
; CHECK-NEXT: [[SUB:%.*]] = sub nsw i32 0, [[X]]
; CHECK-NEXT: [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
; CHECK-NEXT: ret i32 [[COND1]]
; CHECK-NEXT: [[COND:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
; CHECK-NEXT: ret i32 [[COND]]
;
%cmp = icmp sgt i32 %x, 0
%sub = sub nsw i32 0, %x
@ -965,8 +965,8 @@ define i32 @nabs_abs_x11(i32 %x) {
; CHECK-LABEL: @nabs_abs_x11(
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
; CHECK-NEXT: [[SUB:%.*]] = sub nsw i32 0, [[X]]
; CHECK-NEXT: [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
; CHECK-NEXT: ret i32 [[COND1]]
; CHECK-NEXT: [[COND:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
; CHECK-NEXT: ret i32 [[COND]]
;
%cmp = icmp slt i32 %x, 0
%sub = sub nsw i32 0, %x
@ -981,8 +981,8 @@ define i32 @nabs_abs_x12(i32 %x) {
; CHECK-LABEL: @nabs_abs_x12(
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
; CHECK-NEXT: [[SUB:%.*]] = sub nsw i32 0, [[X]]
; CHECK-NEXT: [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
; CHECK-NEXT: ret i32 [[COND1]]
; CHECK-NEXT: [[COND:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
; CHECK-NEXT: ret i32 [[COND]]
;
%cmp = icmp slt i32 %x, 1
%sub = sub nsw i32 0, %x