[PATCH] D47041: [ValueTracking] Teach computeKnownBits that the result of an absolute value pattern that uses nsw flag is always positive.
Craig Topper via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Tue May 22 14:59:26 PDT 2018
craig.topper updated this revision to Diff 148111.
craig.topper added a comment.
Add a combine to negate ABS/NABS to recover the broken tests.
https://reviews.llvm.org/D47041
Files:
lib/Analysis/ValueTracking.cpp
lib/Transforms/InstCombine/InstCombineAddSub.cpp
test/Transforms/InstCombine/abs_abs.ll
Index: test/Transforms/InstCombine/abs_abs.ll
===================================================================
--- test/Transforms/InstCombine/abs_abs.ll
+++ test/Transforms/InstCombine/abs_abs.ll
@@ -933,8 +933,8 @@
; 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: [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT: ret i32 [[SUB16]]
;
%cmp = icmp sgt i32 %x, -1
%sub = sub nsw i32 0, %x
@@ -949,8 +949,8 @@
; 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: [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT: ret i32 [[SUB16]]
;
%cmp = icmp sgt i32 %x, 0
%sub = sub nsw i32 0, %x
@@ -965,8 +965,8 @@
; 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: [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT: ret i32 [[SUB16]]
;
%cmp = icmp slt i32 %x, 0
%sub = sub nsw i32 0, %x
@@ -981,8 +981,8 @@
; 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: [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT: ret i32 [[SUB16]]
;
%cmp = icmp slt i32 %x, 1
%sub = sub nsw i32 0, %x
Index: lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1628,6 +1628,18 @@
Value *ShAmtOp = cast<Instruction>(Op1)->getOperand(1);
return BinaryOperator::CreateLShr(X, ShAmtOp);
}
+
+ if (Op1->hasOneUse()) {
+ Value *LHS, *RHS;
+ SelectPatternFlavor SPF = matchSelectPattern(Op1, LHS, RHS).Flavor;
+ if (SPF == SPF_ABS || SPF == SPF_NABS) {
+ // This is a negate of an ABS/NABS pattern. Just swap the operands
+ // of the select.
+ SelectInst *SI = cast<SelectInst>(Op1);
+ return SelectInst::Create(SI->getCondition(), SI->getFalseValue(),
+ SI->getTrueValue());
+ }
+ }
}
// Turn this into a xor if LHS is 2^n-1 and the remaining bits are known
Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -1078,6 +1078,12 @@
// 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.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: D47041.148111.patch
Type: text/x-patch
Size: 3689 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20180522/04a2d6c7/attachment.bin>
More information about the llvm-commits
mailing list