llvm dev - Jun 2020 - InstCombine doesn't delete instructions with token

Hello David,

I am having an issue with some custom intrinsics that return a token value:
InstCombine deletes the users of the token but not the instruction that
creates the token itself. The IR is still valid but it's wasted.

The source of the issue is coming from an old patch of yours:

commit 7204cff0a121ebc770cf81f7f94679ae7324daae
Author: David Majnemer <david.majnemer at gmail.com>
Date:   Fri Nov 6 21:26:32 2015 +0000

    [InstCombine] Don't RAUW tokens with undef

    Let SimplifyCFG remove unreachable BBs which define token instructions.

    llvm-svn: 252343

--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3012,7 +3012,7 @@ static bool prepareICWorklistFromFunction(Function
&F, const DataLayout &DL,
     while (EndInst != BB->begin()) {
       // Delete the next to last instruction.
       Instruction *Inst = &*--EndInst->getIterator();
-      if (!Inst->use_empty())
+      if (!Inst->use_empty() && !Inst->getType()->isTokenTy())
         Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
-      if (Inst->isEHPad()) {
+      if (Inst->isEHPad() || Inst->getType()->isTokenTy()) {
         EndInst = Inst;
@@ -3022,7 +3022,8 @@ static bool prepareICWorklistFromFunction(Function
&F, const DataLayout &DL,
         ++NumDeadInst;
         MadeIRChange = true;
       }
       Inst->eraseFromParent();
     }
   }

I believe the goal was to avoid RAUW the EHPad (as we don't delete the
associated EHRet) and not skip all of the token instructions. At least you
only test on EHPad in the associated unit test.

In which case we could instead do:

     while (EndInst != BB->begin()) {
       // Delete the next to last instruction.
       Instruction *Inst = &*--EndInst->getIterator();
-      if (!Inst->use_empty())
+      if (!Inst->use_empty() && !Inst->isEHPad())
         Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
       if (Inst->isEHPad()) {
         EndInst = Inst;
@@ -3022,7 +3022,8 @@ static bool prepareICWorklistFromFunction(Function
&F, const DataLayout &DL,
         ++NumDeadInst;
         MadeIRChange = true;
       }
       Inst->eraseFromParent();
     }

Is my assumption correct?

Note that the code is now in
llvm::removeAllNonTerminatorAndEHPadInstructions of
llvm/lib/Transforms/Utils/Local.cpp

-- 
*Alexandre Isoard*
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In general, we have to RAUW before we erase an instruction in dead code; even if
we know the instruction is dead, it could still have uses in other dead code. 
If an instruction still has uses, we can’t erase it.

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Alexandre
Isoard via llvm-dev
Sent: Tuesday, June 16, 2020 7:33 PM
To: David Majnemer <david.majnemer at gmail.com>; llvm-dev <llvm-dev at
lists.llvm.org>
Subject: [EXT] [llvm-dev] InstCombine doesn't delete instructions with token

Hello David,

I am having an issue with some custom intrinsics that return a token value:
InstCombine deletes the users of the token but not the instruction that creates
the token itself. The IR is still valid but it's wasted.

The source of the issue is coming from an old patch of yours:

commit 7204cff0a121ebc770cf81f7f94679ae7324daae
Author: David Majnemer <david.majnemer at gmail.com<mailto:david.majnemer
at gmail.com>>
Date:   Fri Nov 6 21:26:32 2015 +0000

    [InstCombine] Don't RAUW tokens with undef

    Let SimplifyCFG remove unreachable BBs which define token instructions.

    llvm-svn: 252343

--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3012,7 +3012,7 @@ static bool prepareICWorklistFromFunction(Function &F,
const DataLayout &DL,
     while (EndInst != BB->begin()) {
       // Delete the next to last instruction.
       Instruction *Inst = &*--EndInst->getIterator();
-      if (!Inst->use_empty())
+      if (!Inst->use_empty() && !Inst->getType()->isTokenTy())
         Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
-      if (Inst->isEHPad()) {
+      if (Inst->isEHPad() || Inst->getType()->isTokenTy()) {
         EndInst = Inst;
@@ -3022,7 +3022,8 @@ static bool prepareICWorklistFromFunction(Function &F,
const DataLayout &DL,
         ++NumDeadInst;
         MadeIRChange = true;
       }
       Inst->eraseFromParent();
     }
   }

I believe the goal was to avoid RAUW the EHPad (as we don't delete the
associated EHRet) and not skip all of the token instructions. At least you only
test on EHPad in the associated unit test.

In which case we could instead do:

     while (EndInst != BB->begin()) {
       // Delete the next to last instruction.
       Instruction *Inst = &*--EndInst->getIterator();
-      if (!Inst->use_empty())
+      if (!Inst->use_empty() && !Inst->isEHPad())
         Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
       if (Inst->isEHPad()) {
         EndInst = Inst;
@@ -3022,7 +3022,8 @@ static bool prepareICWorklistFromFunction(Function &F,
const DataLayout &DL,
         ++NumDeadInst;
         MadeIRChange = true;
       }
       Inst->eraseFromParent();
     }

Is my assumption correct?

Note that the code is now in llvm::removeAllNonTerminatorAndEHPadInstructions of
llvm/lib/Transforms/Utils/Local.cpp

--
Alexandre Isoard
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Yes, it's still respected in this case, as the only instructions that will
be deleted have been RAUW with undef.

Originally, all instructions where RAUW but only non-EHPad were deleted
(that means EHPad were RAUW but not deleted).
Then it was later patched by not RAUW token instructions and now not
deleting EHPad nor token instructions.

My assumption is that the instructions we wanted to avoid RAUW were only
the EHPad, not all the token instructions.
So I'm changing it into RAUW all non-EHPad instructions and delete all
non-EHPad instructions.

But maybe my assumption is incorrect and there are token instructions that
are non-EHPad that we want to skip too?

On Tue, Jun 16, 2020 at 7:55 PM Eli Friedman <efriedma at quicinc.com>
wrote:
> In general, we have to RAUW before we erase an instruction in dead code;
> even if we know the instruction is dead, it could still have uses in other
> dead code.  If an instruction still has uses, we can’t erase it.
>
>
>
> -Eli
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of
*Alexandre
> Isoard via llvm-dev
> *Sent:* Tuesday, June 16, 2020 7:33 PM
> *To:* David Majnemer <david.majnemer at gmail.com>; llvm-dev <
> llvm-dev at lists.llvm.org>
> *Subject:* [EXT] [llvm-dev] InstCombine doesn't delete instructions
with
> token
>
>
>
> Hello David,
>
>
>
> I am having an issue with some custom intrinsics that return a token
> value: InstCombine deletes the users of the token but not the instruction
> that creates the token itself. The IR is still valid but it's wasted.
>
>
>
> The source of the issue is coming from an old patch of yours:
>
>
>
> commit 7204cff0a121ebc770cf81f7f94679ae7324daae
> Author: David Majnemer <david.majnemer at gmail.com>
> Date:   Fri Nov 6 21:26:32 2015 +0000
>
>     [InstCombine] Don't RAUW tokens with undef
>
>     Let SimplifyCFG remove unreachable BBs which define token instructions.
>
>     llvm-svn: 252343
>
> --- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
> +++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
> @@ -3012,7 +3012,7 @@ static bool prepareICWorklistFromFunction(Function
> &F, const DataLayout &DL,
>      while (EndInst != BB->begin()) {
>        // Delete the next to last instruction.
>        Instruction *Inst = &*--EndInst->getIterator();
> -      if (!Inst->use_empty())
> +      if (!Inst->use_empty() &&
!Inst->getType()->isTokenTy())
>          Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
> -      if (Inst->isEHPad()) {
>
> +      if (Inst->isEHPad() || Inst->getType()->isTokenTy()) {
>          EndInst = Inst;
> @@ -3022,7 +3022,8 @@ static bool prepareICWorklistFromFunction(Function
> &F, const DataLayout &DL,
>          ++NumDeadInst;
>          MadeIRChange = true;
>        }
>        Inst->eraseFromParent();
>      }
>    }
>
>
>
> I believe the goal was to avoid RAUW the EHPad (as we don't delete the
> associated EHRet) and not skip all of the token instructions. At least you
> only test on EHPad in the associated unit test.
>
>
>
> In which case we could instead do:
>
>
>
>      while (EndInst != BB->begin()) {
>        // Delete the next to last instruction.
>        Instruction *Inst = &*--EndInst->getIterator();
> -      if (!Inst->use_empty())
> +      if (!Inst->use_empty() && !Inst->isEHPad())
>          Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
>        if (Inst->isEHPad()) {
>
>          EndInst = Inst;
> @@ -3022,7 +3022,8 @@ static bool prepareICWorklistFromFunction(Function
> &F, const DataLayout &DL,
>          ++NumDeadInst;
>          MadeIRChange = true;
>        }
>        Inst->eraseFromParent();
>      }
>
>
>
> Is my assumption correct?
>
>
>
> Note that the code is now in
> llvm::removeAllNonTerminatorAndEHPadInstructions of
> llvm/lib/Transforms/Utils/Local.cpp
>
>
>
> --
>
> *Alexandre Isoard*
>

-- 
*Alexandre Isoard*
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