[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
Wed May 23 10:43:59 PDT 2018
craig.topper updated this revision to Diff 148247.
craig.topper added a comment.
Rebase to remvoe the InstCombine changes that were split off to another patch and have now been committed
https://reviews.llvm.org/D47041
Files:
lib/Analysis/ValueTracking.cpp
test/Transforms/InstCombine/abs-1.ll
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: test/Transforms/InstCombine/abs-1.ll
===================================================================
--- test/Transforms/InstCombine/abs-1.ll
+++ test/Transforms/InstCombine/abs-1.ll
@@ -293,3 +293,14 @@
%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
+}
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.
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