[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 22:02:17 PDT 2016
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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