[llvm] r309758 - [SCEV/IndVars] Always compute loop exiting values if the backedge count is 0

[Davide Italiano via llvm-commits]

Sanjoy, Hans, can we backport this one to 5.0?

Thanks,

--
Davide

On Tue, Aug 1, 2017 at 3:37 PM, Sanjoy Das via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: sanjoy
> Date: Tue Aug  1 15:37:58 2017
> New Revision: 309758
>
> URL: http://llvm.org/viewvc/llvm-project?rev=309758&view=rev
> Log:
> [SCEV/IndVars] Always compute loop exiting values if the backedge count is 0
>
> If SCEV can prove that the backedge taken count for a loop is zero, it does not
> need to "understand" a recursive PHI to compute its exiting value.
>
> This should fix PR33885.
>
> Modified:
>     llvm/trunk/lib/Analysis/ScalarEvolution.cpp
>     llvm/trunk/test/Transforms/IndVarSimplify/exit_value_test2.ll
>
> Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=309758&r1=309757&r2=309758&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
> +++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Tue Aug  1 15:37:58 2017
> @@ -7597,6 +7597,25 @@ const SCEV *ScalarEvolution::computeSCEV
>              const SCEV *BackedgeTakenCount = getBackedgeTakenCount(LI);
>              if (const SCEVConstant *BTCC =
>                    dyn_cast<SCEVConstant>(BackedgeTakenCount)) {
> +
> +              // This trivial case can show up in some degenerate cases where
> +              // the incoming IR has not yet been fully simplified.
> +              if (BTCC->getValue()->isZero()) {
> +                Value *InitValue = nullptr;
> +                bool MultipleInitValues = false;
> +                for (unsigned i = 0; i < PN->getNumIncomingValues(); i++) {
> +                  if (!LI->contains(PN->getIncomingBlock(i))) {
> +                    if (!InitValue)
> +                      InitValue = PN->getIncomingValue(i);
> +                    else if (InitValue != PN->getIncomingValue(i)) {
> +                      MultipleInitValues = true;
> +                      break;
> +                    }
> +                  }
> +                  if (!MultipleInitValues && InitValue)
> +                    return getSCEV(InitValue);
> +                }
> +              }
>                // Okay, we know how many times the containing loop executes.  If
>                // this is a constant evolving PHI node, get the final value at
>                // the specified iteration number.
>
> Modified: llvm/trunk/test/Transforms/IndVarSimplify/exit_value_test2.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IndVarSimplify/exit_value_test2.ll?rev=309758&r1=309757&r2=309758&view=diff
> ==============================================================================
> --- llvm/trunk/test/Transforms/IndVarSimplify/exit_value_test2.ll (original)
> +++ llvm/trunk/test/Transforms/IndVarSimplify/exit_value_test2.ll Tue Aug  1 15:37:58 2017
> @@ -3,15 +3,14 @@
>
>  ; Check IndVarSimplify should not replace exit value because or else
>  ; udiv will be introduced by expand and the cost will be high.
> -;
> -; CHECK-LABEL: @_Z3fooPKcjj(
> -; CHECK-NOT: udiv
>
>  declare void @_Z3mixRjj(i32* dereferenceable(4), i32)
>  declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture)
>  declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture)
>
>  define i32 @_Z3fooPKcjj(i8* nocapture readonly %s, i32 %len, i32 %c) {
> +; CHECK-LABEL: @_Z3fooPKcjj(
> +; CHECK-NOT: udiv
>  entry:
>    %a = alloca i32, align 4
>    %tmp = bitcast i32* %a to i8*
> @@ -50,3 +49,26 @@ while.end:
>    call void @llvm.lifetime.end.p0i8(i64 4, i8* %tmp)
>    ret i32 %tmp4
>  }
> +
> +define i32 @zero_backedge_count_test(i32 %unknown_init, i32* %unknown_mem) {
> +; CHECK-LABEL: @zero_backedge_count_test(
> +entry:
> +  br label %loop
> +
> +loop:
> +  %iv = phi i32 [ 0, %entry], [ %iv.inc, %loop ]
> +  %unknown_phi = phi i32 [ %unknown_init, %entry ], [ %unknown_next, %loop ]
> +  %iv.inc = add i32 %iv, 1
> +  %be_taken = icmp ne i32 %iv.inc, 1
> +  %unknown_next = load volatile i32, i32* %unknown_mem
> +  br i1 %be_taken, label %loop, label %leave
> +
> +leave:
> +; We can fold %unknown_phi even though the backedge value for it is completely
> +; unknown, since we can prove that the loop's backedge taken count is 0.
> +
> +; CHECK: leave:
> +; CHECK: ret i32 %unknown_init
> +  %exit_val = phi i32 [ %unknown_phi, %loop ]
> +  ret i32 %exit_val
> +}
>
>
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-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare