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[InstSimplify] shl nuw C, %x -> C iff signbit is set on C. 

Summary:
`%r = shl nuw i8 C, %x`

As per langref:
```
If the nuw keyword is present, then the shift produces
a poison value if it shifts out any non-zero bits.
```
Thus, if the sign bit is set on `C`, then `%x` can only be `0`,
which means that `%r` can only be `C`.
Or in other words, set sign bit means that the signed value
is negative, so the constant is `<= 0`.

https://rise4fun.com/Alive/WMk
https://rise4fun.com/Alive/udv

Was mentioned in D47428 review.

We already handle the `0` constant, https://godbolt.org/g/UZq1sJ, so this only handles negative constants.

Could use computeKnownBits() / LazyValueInfo,
but the cost-benefit analysis (https://reviews.llvm.org/D47891)
suggests it isn't worth it.

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47883

llvm-svn: 334222
This commit is contained in:
Roman Lebedev 2018-06-07 20:03:45 +00:00
parent 1f67a3cba9
commit 2683802ba0
3 changed files with 21 additions and 36 deletions
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7
llvm/lib/Analysis/InstructionSimplify.cpp
View File

@ -1226,6 +1226,13 @@ static Value *SimplifyShlInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
Value *X;
if (match(Op0, m_Exact(m_Shr(m_Value(X), m_Specific(Op1)))))
return X;
// shl nuw i8 C, %x -> C iff C has sign bit set.
if (isNUW && match(Op0, m_Negative()))
return Op0;
// NOTE: could use computeKnownBits() / LazyValueInfo,
// but the cost-benefit analysis suggests it isn't worth it.
return nullptr;
}

32
llvm/test/Transforms/InstCombine/set-lowbits-mask-canonicalize.ll
View File

@ -39,9 +39,7 @@ define i32 @shl_add_nsw(i32 %NBits) {
define i32 @shl_add_nuw(i32 %NBits) {
; CHECK-LABEL: @shl_add_nuw(
; CHECK-NEXT: [[NOTMASK:%.*]] = shl nuw nsw i32 -1, [[NBITS:%.*]]
; CHECK-NEXT: [[RET:%.*]] = xor i32 [[NOTMASK]], -1
; CHECK-NEXT: ret i32 [[RET]]
; CHECK-NEXT: ret i32 0
;
%setbit = shl i32 1, %NBits
%ret = add nuw i32 %setbit, -1
@ -50,9 +48,7 @@ define i32 @shl_add_nuw(i32 %NBits) {
define i32 @shl_add_nsw_nuw(i32 %NBits) {
; CHECK-LABEL: @shl_add_nsw_nuw(
; CHECK-NEXT: [[NOTMASK:%.*]] = shl nuw nsw i32 -1, [[NBITS:%.*]]
; CHECK-NEXT: [[RET:%.*]] = xor i32 [[NOTMASK]], -1
; CHECK-NEXT: ret i32 [[RET]]
; CHECK-NEXT: ret i32 0
;
%setbit = shl i32 1, %NBits
%ret = add nuw nsw i32 %setbit, -1
@ -85,9 +81,7 @@ define i32 @shl_nsw_add_nsw(i32 %NBits) {
define i32 @shl_nsw_add_nuw(i32 %NBits) {
; CHECK-LABEL: @shl_nsw_add_nuw(
; CHECK-NEXT: [[NOTMASK:%.*]] = shl nuw nsw i32 -1, [[NBITS:%.*]]
; CHECK-NEXT: [[RET:%.*]] = xor i32 [[NOTMASK]], -1
; CHECK-NEXT: ret i32 [[RET]]
; CHECK-NEXT: ret i32 0
;
%setbit = shl nsw i32 1, %NBits
%ret = add nuw i32 %setbit, -1
@ -96,9 +90,7 @@ define i32 @shl_nsw_add_nuw(i32 %NBits) {
define i32 @shl_nsw_add_nsw_nuw(i32 %NBits) {
; CHECK-LABEL: @shl_nsw_add_nsw_nuw(
; CHECK-NEXT: [[NOTMASK:%.*]] = shl nuw nsw i32 -1, [[NBITS:%.*]]
; CHECK-NEXT: [[RET:%.*]] = xor i32 [[NOTMASK]], -1
; CHECK-NEXT: ret i32 [[RET]]
; CHECK-NEXT: ret i32 0
;
%setbit = shl nsw i32 1, %NBits
%ret = add nuw nsw i32 %setbit, -1
@ -131,9 +123,7 @@ define i32 @shl_nuw_add_nsw(i32 %NBits) {
define i32 @shl_nuw_add_nuw(i32 %NBits) {
; CHECK-LABEL: @shl_nuw_add_nuw(
; CHECK-NEXT: [[NOTMASK:%.*]] = shl nuw nsw i32 -1, [[NBITS:%.*]]
; CHECK-NEXT: [[RET:%.*]] = xor i32 [[NOTMASK]], -1
; CHECK-NEXT: ret i32 [[RET]]
; CHECK-NEXT: ret i32 0
;
%setbit = shl nuw i32 1, %NBits
%ret = add nuw i32 %setbit, -1
@ -142,9 +132,7 @@ define i32 @shl_nuw_add_nuw(i32 %NBits) {
define i32 @shl_nuw_add_nsw_nuw(i32 %NBits) {
; CHECK-LABEL: @shl_nuw_add_nsw_nuw(
; CHECK-NEXT: [[NOTMASK:%.*]] = shl nuw nsw i32 -1, [[NBITS:%.*]]
; CHECK-NEXT: [[RET:%.*]] = xor i32 [[NOTMASK]], -1
; CHECK-NEXT: ret i32 [[RET]]
; CHECK-NEXT: ret i32 0
;
%setbit = shl nuw i32 1, %NBits
%ret = add nuw nsw i32 %setbit, -1
@ -177,9 +165,7 @@ define i32 @shl_nsw_nuw_add_nsw(i32 %NBits) {
define i32 @shl_nsw_nuw_add_nuw(i32 %NBits) {
; CHECK-LABEL: @shl_nsw_nuw_add_nuw(
; CHECK-NEXT: [[NOTMASK:%.*]] = shl nuw nsw i32 -1, [[NBITS:%.*]]
; CHECK-NEXT: [[RET:%.*]] = xor i32 [[NOTMASK]], -1
; CHECK-NEXT: ret i32 [[RET]]
; CHECK-NEXT: ret i32 0
;
%setbit = shl nuw nsw i32 1, %NBits
%ret = add nuw i32 %setbit, -1
@ -188,9 +174,7 @@ define i32 @shl_nsw_nuw_add_nuw(i32 %NBits) {
define i32 @shl_nsw_nuw_add_nsw_nuw(i32 %NBits) {
; CHECK-LABEL: @shl_nsw_nuw_add_nsw_nuw(
; CHECK-NEXT: [[NOTMASK:%.*]] = shl nuw nsw i32 -1, [[NBITS:%.*]]
; CHECK-NEXT: [[RET:%.*]] = xor i32 [[NOTMASK]], -1
; CHECK-NEXT: ret i32 [[RET]]
; CHECK-NEXT: ret i32 0
;
%setbit = shl nuw nsw i32 1, %NBits
%ret = add nuw nsw i32 %setbit, -1

18
llvm/test/Transforms/InstSimplify/constantfold-shl-nuw-C-to-C.ll
View File

@ -9,8 +9,7 @@
define i8 @shl_nuw (i8 %x) {
; CHECK-LABEL: @shl_nuw(
; CHECK-NEXT: [[RET:%.*]] = shl nuw i8 -1, [[X:%.*]]
; CHECK-NEXT: ret i8 [[RET]]
; CHECK-NEXT: ret i8 -1
;
%ret = shl nuw i8 -1, %x
; nuw here means that %x can only be 0
@ -19,8 +18,7 @@ define i8 @shl_nuw (i8 %x) {
define i8 @shl_nuw_nsw (i8 %x) {
; CHECK-LABEL: @shl_nuw_nsw(
; CHECK-NEXT: [[RET:%.*]] = shl nuw nsw i8 -1, [[X:%.*]]
; CHECK-NEXT: ret i8 [[RET]]
; CHECK-NEXT: ret i8 -1
;
%ret = shl nuw nsw i8 -1, %x
; nuw here means that %x can only be 0
@ -29,8 +27,7 @@ define i8 @shl_nuw_nsw (i8 %x) {
define i8 @shl_128 (i8 %x) {
; CHECK-LABEL: @shl_128(
; CHECK-NEXT: [[RET:%.*]] = shl nuw i8 -128, [[X:%.*]]
; CHECK-NEXT: ret i8 [[RET]]
; CHECK-NEXT: ret i8 -128
;
%ret = shl nuw i8 128, %x
; 128 == 1<<7 == just the sign bit is set
@ -75,8 +72,7 @@ define i8 @knownbits_negativeorzero(i8 %x, i8 %y) {
define <2 x i8> @shl_vec(<2 x i8> %x) {
; CHECK-LABEL: @shl_vec(
; CHECK-NEXT: [[RET:%.*]] = shl nuw <2 x i8> <i8 -1, i8 -1>, [[X:%.*]]
; CHECK-NEXT: ret <2 x i8> [[RET]]
; CHECK-NEXT: ret <2 x i8> <i8 -1, i8 -1>
;
%ret = shl nuw <2 x i8> <i8 -1, i8 -1>, %x
ret <2 x i8> %ret
@ -84,8 +80,7 @@ define <2 x i8> @shl_vec(<2 x i8> %x) {
define <3 x i8> @shl_vec_undef(<3 x i8> %x) {
; CHECK-LABEL: @shl_vec_undef(
; CHECK-NEXT: [[RET:%.*]] = shl nuw <3 x i8> <i8 -1, i8 undef, i8 -1>, [[X:%.*]]
; CHECK-NEXT: ret <3 x i8> [[RET]]
; CHECK-NEXT: ret <3 x i8> <i8 -1, i8 undef, i8 -1>
;
%ret = shl nuw <3 x i8> <i8 -1, i8 undef, i8 -1>, %x
ret <3 x i8> %ret
@ -93,8 +88,7 @@ define <3 x i8> @shl_vec_undef(<3 x i8> %x) {
define <2 x i8> @shl_vec_nonsplat(<2 x i8> %x) {
; CHECK-LABEL: @shl_vec_nonsplat(
; CHECK-NEXT: [[RET:%.*]] = shl nuw <2 x i8> <i8 -1, i8 -2>, [[X:%.*]]
; CHECK-NEXT: ret <2 x i8> [[RET]]
; CHECK-NEXT: ret <2 x i8> <i8 -1, i8 -2>
;
%ret = shl nuw <2 x i8> <i8 -1, i8 -2>, %x
ret <2 x i8> %ret