[llvm] r274098 - [ValueTracking] Teach computeKnownBits for PHI nodes to compute sign bit for a recurrence with a NSW addition.
Craig Topper via llvm-commits
llvm-commits at lists.llvm.org
Tue Jun 28 22:28:08 PDT 2016
My change seems to have caused an "and" with 4294967295 to be inserted into
some of the getelementptrs. A similar thing happens if you change the type
of "i" in the loop to "unsigned". Though "unsigned" causes ands on even
more of the getelementptrs.
~Craig
On Tue, Jun 28, 2016 at 10:14 PM, Adam Nemet <anemet at apple.com> wrote:
> As I said in the review, my worry is that we could be regressing some
> cases that we used to vectorize. I’d like to see some analysis why this
> happened, i.e. why we were able to get the array bounds before. Is it
> because we’re now widening some IV and the truncs prevent us forming nice
> SCEV add-recurrences? (We can only get bounds for add-recurrences.)
>
> Adam
>
>
> On Jun 28, 2016, at 10:02 PM, Craig Topper <craig.topper at gmail.com> wrote:
>
> I see that. Looks like changing N to a constant or starting the loop at -1
> fixes my case. Changing indices to all +1 like D21773 does not fix it. What
> do you suggest?
>
> ~Craig
>
> On Tue, Jun 28, 2016 at 9:56 PM, Adam Nemet <anemet at apple.com> wrote:
>
>> Looks like this broke the same test the same way that we’ve been
>> discussing under http://reviews.llvm.org/D21773
>>
>> Adam
>>
>> > On Jun 28, 2016, at 8:46 PM, Craig Topper via llvm-commits <
>> llvm-commits at lists.llvm.org> wrote:
>> >
>> > Author: ctopper
>> > Date: Tue Jun 28 22:46:47 2016
>> > New Revision: 274098
>> >
>> > URL: http://llvm.org/viewvc/llvm-project?rev=274098&view=rev
>> > Log:
>> > [ValueTracking] Teach computeKnownBits for PHI nodes to compute sign
>> bit for a recurrence with a NSW addition.
>> >
>> > If a operation for a recurrence is an addition with no signed wrap and
>> both input sign bits are 0, then the result sign bit must also be 0.
>> Similar for the negative case.
>> >
>> > I found this deficiency while playing around with a loop in the x86
>> backend that contained a signed division that could be optimized into an
>> unsigned division if we could prove both inputs were positive. One of them
>> being the loop induction variable. With this patch we can perform the
>> conversion for this case. One of the test cases here is a contrived
>> variation of the loop I was looking at.
>> >
>> > Differential revision: http://reviews.llvm.org/D21493
>> >
>> > Modified:
>> > llvm/trunk/lib/Analysis/ValueTracking.cpp
>> > llvm/trunk/test/Transforms/BBVectorize/loop1.ll
>> > llvm/trunk/test/Transforms/InstCombine/phi.ll
>> >
>> > Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
>> > URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=274098&r1=274097&r2=274098&view=diff
>> >
>> ==============================================================================
>> > --- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
>> > +++ llvm/trunk/lib/Analysis/ValueTracking.cpp Tue Jun 28 22:46:47 2016
>> > @@ -1240,6 +1240,18 @@ static void computeKnownBitsFromOperator
>> > KnownZero = APInt::getLowBitsSet(BitWidth,
>> >
>> std::min(KnownZero2.countTrailingOnes(),
>> >
>> KnownZero3.countTrailingOnes()));
>> > +
>> > + // If the operation is an addition that can't have signed
>> overflow,
>> > + // then the sign bit is known to be zero if both input sign
>> bits
>> > + // are zero. Similar for two negative inputs.
>> > + if (Opcode == Instruction::Add &&
>> > + cast<OverflowingBinaryOperator>(LU)->hasNoSignedWrap()) {
>> > + if (KnownZero2.isNegative() && KnownZero3.isNegative())
>> > + KnownZero.setBit(BitWidth-1);
>> > + if (KnownOne2.isNegative() && KnownOne3.isNegative())
>> > + KnownOne.setBit(BitWidth-1);
>> > + }
>> > +
>> > break;
>> > }
>> > }
>> >
>> > Modified: llvm/trunk/test/Transforms/BBVectorize/loop1.ll
>> > URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/BBVectorize/loop1.ll?rev=274098&r1=274097&r2=274098&view=diff
>> >
>> ==============================================================================
>> > --- llvm/trunk/test/Transforms/BBVectorize/loop1.ll (original)
>> > +++ llvm/trunk/test/Transforms/BBVectorize/loop1.ll Tue Jun 28 22:46:47
>> 2016
>> > @@ -83,7 +83,7 @@ for.body:
>> > ; CHECK-UNRL: %add12 = fadd <2 x double> %add7, %mul11
>> > ; CHECK-UNRL: %4 = bitcast double* %arrayidx14 to <2 x double>*
>> > ; CHECK-UNRL: store <2 x double> %add12, <2 x double>* %4, align 8
>> > -; CHECK-UNRL: %indvars.iv.next.1 = add nsw i64 %indvars.iv, 2
>> > +; CHECK-UNRL: %indvars.iv.next.1 = add nuw nsw i64 %indvars.iv, 2
>> > ; CHECK-UNRL: %lftr.wideiv.1 = trunc i64 %indvars.iv.next.1 to i32
>> > ; CHECK-UNRL: %exitcond.1 = icmp eq i32 %lftr.wideiv.1, 10
>> > ; CHECK-UNRL: br i1 %exitcond.1, label %for.end, label %for.body
>> >
>> > Modified: llvm/trunk/test/Transforms/InstCombine/phi.ll
>> > URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/phi.ll?rev=274098&r1=274097&r2=274098&view=diff
>> >
>> ==============================================================================
>> > --- llvm/trunk/test/Transforms/InstCombine/phi.ll (original)
>> > +++ llvm/trunk/test/Transforms/InstCombine/phi.ll Tue Jun 28 22:46:47
>> 2016
>> > @@ -879,3 +879,118 @@ if.end:
>> > %cmp1 = icmp ne i32 %a.0, 0
>> > ret i1 %cmp1
>> > }
>> > +
>> > +
>> > +; This test makes sure we can determine that the inputs to the sdiv in
>> the loop
>> > +; are non-negative and can become a udiv. This requires that we
>> recognize that
>> > +; the loop induction can never have its sign bit set.
>> > +;
>> > +; CHECK-LABEL: @phi_nsw_induction_sdiv_udiv
>> > +; CHECK: udiv
>> > +; CHECK: udiv
>> > +define i32 @phi_nsw_induction_sdiv_udiv(i32 %NumElts, i32 %ScalarSize)
>> {
>> > +entry:
>> > + %div = udiv i32 128, %ScalarSize
>> > + br label %for.cond
>> > +
>> > +for.cond: ; preds = %for.inc,
>> %entry
>> > + %Sum.0 = phi i32 [ 0, %entry ], [ %add, %for.inc ]
>> > + %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
>> > + %cmp = icmp ne i32 %i.0, %NumElts
>> > + br i1 %cmp, label %for.body, label %for.end
>> > +
>> > +for.body: ; preds = %for.cond
>> > + ; this should become a udiv
>> > + %div1 = sdiv i32 %i.0, %div
>> > + %add = add nsw i32 %Sum.0, %div1
>> > + br label %for.inc
>> > +
>> > +for.inc: ; preds = %for.body
>> > + %inc = add nsw i32 %i.0, 1
>> > + br label %for.cond
>> > +
>> > +for.end: ; preds = %for.cond
>> > + ret i32 %Sum.0
>> > +}
>> > +
>> > +
>> > +; CHECK-LABEL: test_positive_nsw_recurrence
>> > +; CHECK-NOT: bar
>> > +; CHECK: foo
>> > +; CHECK-NOT: bar
>> > +; CHECK: ret
>> > +define void @test_positive_nsw_recurrence(i32 %N) {
>> > +entry:
>> > + br label %for.cond
>> > +
>> > +for.cond: ; preds = %for.inc,
>> %entry
>> > + %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
>> > + %cmp = icmp ne i32 %i.0, %N
>> > + br i1 %cmp, label %for.body, label %for.end
>> > +
>> > +for.body: ; preds = %for.cond
>> > + %and = and i32 %i.0, -2147483648
>> > + %tobool = icmp ne i32 %and, 0
>> > + br i1 %tobool, label %if.then, label %if.else
>> > +
>> > +if.then: ; preds = %for.body
>> > + ; this call should be deleted as %i.0 can never be negative due to
>> no signed wrap
>> > + call void @bar()
>> > + br label %if.end
>> > +
>> > +if.else: ; preds = %for.body
>> > + call void @foo()
>> > + br label %if.end
>> > +
>> > +if.end: ; preds = %if.else,
>> %if.then
>> > + br label %for.inc
>> > +
>> > +for.inc: ; preds = %if.end
>> > + %inc = add nsw i32 %i.0, 1
>> > + br label %for.cond
>> > +
>> > +for.end: ; preds = %for.cond
>> > + ret void
>> > +}
>> > +
>> > +; CHECK-LABEL: test_negative_nsw_recurrence
>> > +; CHECK-NOT: foo
>> > +; CHECK: bar
>> > +; CHECK-NOT: foo
>> > +; CHECK: ret
>> > +define void @test_negative_nsw_recurrence(i32 %N) {
>> > +entry:
>> > + br label %for.cond
>> > +
>> > +for.cond: ; preds = %for.inc,
>> %entry
>> > + %i.0 = phi i32 [ -1, %entry ], [ %inc, %for.inc ]
>> > + %cmp = icmp ne i32 %i.0, %N
>> > + br i1 %cmp, label %for.body, label %for.end
>> > +
>> > +for.body: ; preds = %for.cond
>> > + %and = and i32 %i.0, -2147483648
>> > + %tobool = icmp ne i32 %and, 0
>> > + br i1 %tobool, label %if.then, label %if.else
>> > +
>> > +if.then: ; preds = %for.body
>> > + call void @bar()
>> > + br label %if.end
>> > +
>> > +if.else: ; preds = %for.body
>> > + ; this call should be deleted as %i.0 can never be positive due to
>> no signed wrap
>> > + call void @foo()
>> > + br label %if.end
>> > +
>> > +if.end: ; preds = %if.else,
>> %if.then
>> > + br label %for.inc
>> > +
>> > +for.inc: ; preds = %if.end
>> > + %inc = add nsw i32 %i.0, -1
>> > + br label %for.cond
>> > +
>> > +for.end: ; preds = %for.cond
>> > + ret void
>> > +}
>> > +
>> > +declare void @bar()
>> > +declare void @foo()
>> >
>> >
>> > _______________________________________________
>> > llvm-commits mailing list
>> > llvm-commits at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>>
>
>
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