[LoopUnswitch] Don't remove instructions with side effects after folding them

[davide]

Summary

This started at

commit 724ecb159395015309d73655641dc56e0063765d
Author: David Majnemer <[e-mail]>
Date:   Tue Jul 19 18:50:26 2016 +0000

    [FunctionAttrs] Correct the safety analysis for inference of 'returned'

    We skipped over ReturnInsts which didn't return an argument which would
    lead us to incorrectly conclude that an argument returned by another
    ReturnInst was 'returned'.

    This reverts commit r275756.

    This fixes PR28610.

The commit exposes, I think, what's a latent bug in LoopUnswitch.
When functionattrs run, we mark the first argument of @fn5 with the returned attribute.

; Function Attrs: noinline nounwind uwtable
define internal fastcc i32 @fn5(i32 returned %ch) unnamed_addr #2 {
entry:

%call = tail call fastcc signext i8 @fn4()
store i8 %call, i8* @c, align 1
tail call fastcc void @fn3()
ret i32 %ch

}

Later on, when LoopUnswitch runs, it calls SimplifyInstruction & if that returns a value, attemps RAUW + eraseFromParent on the instructions in the following block:

for.end: ; preds = %for.cond.for.end_crit_edge, %entry %cj.sroa.0.0.lcssa = phi i8 [ %split, %for.cond.for.end_crit_edge ], [ undef, %entry ] %conv4 = sext i8 %cj.sroa.0.0.lcssa to i32

%call = tail call fastcc i32 @fn5(i32 %conv4)
%conv5 = trunc i32 %call to i16
store i16 %conv5, i16* @ci, align 2                                                                                                                                                  %.pr1 = load i16, i16* @ag, align 2
%tobool = icmp eq i16 %.pr1, 0                                                                                                                                                       br i1 %tobool, label %for.end.while.end.split_crit_edge, label %for.end.for.end.split_crit_edge

This is not quite right I think as we can't DCE an instruction which has side effect, and this is what happens here (the call to @fn5 is DCE'd).

For reference, here's the debug output of the passes with and without the commit:

Replace with '  %cj.sroa.0.03 = phi i8 [ undef, %for.body.lr.ph.split ], [ %.cj.sroa.0.0, %for.body ]':   %.cj.sroa.0.0 = select i1 false, i8 6, i8 %cj.sroa.0.03
Replace with 'i8 undef':   %cj.sroa.0.03 = phi i8 [ undef, %for.body.lr.ph.split ], [ %cj.sroa.0.03, %for.body ]
Replace with 'i8 undef':   %split.ph = phi i8 [ undef, %for.body ]                                                                                                                   
Replace with 'i8 6':   %.cj.sroa.0.0.us = select i1 true, i8 6, i8 %cj.sroa.0.03.us                                                                                                 
Remove dead instruction '  %cj.sroa.0.03.us = phi i8 [ undef, %for.body.lr.ph.split.us ], [ 6, %for.body.us ]
Replace with 'i8 6':   %split.ph.us = phi i8 [ 6, %for.body.us ]        
Replace with 'i8 6':   %split = phi i8 [ undef, %for.cond.for.end_crit_edge.us-lcssa ], [ 6, %for.cond.for.end_crit_edge.us-lcssa.us ]                                               
Replace with 'i8 6':   %cj.sroa.0.0.lcssa = phi i8 [ 6, %for.cond.for.end_crit_edge ], [ undef, %entry ]                                                                             
Replace with 'i32 6':   %conv4 = sext i8 6 to i32                                                                                                                                    
Replace with 'i32 6':   %call = tail call fastcc i32 @fn5(i32 6)                                                                                                                     
Replace with 'i16 6':   %conv5 = trunc i32 6 to i16


Replace with '  %cj.sroa.0.03 = phi i8 [ undef, %for.body.lr.ph.split ], [ %.cj.sroa.0.0, %for.body ]':   %.cj.sroa.0.0 = select i1 false, i8 6, i8 %cj.sroa.0.03                    
Replace with 'i8 undef':   %cj.sroa.0.03 = phi i8 [ undef, %for.body.lr.ph.split ], [ %cj.sroa.0.03, %for.body ]                                                                     
Replace with 'i8 undef':   %split.ph = phi i8 [ undef, %for.body ]                                                                                                                   
Replace with 'i8 6':   %.cj.sroa.0.0.us = select i1 true, i8 6, i8 %cj.sroa.0.03.us                                                                                                 
Remove dead instruction '  %cj.sroa.0.03.us = phi i8 [ undef, %for.body.lr.ph.split.us ], [ 6, %for.body.us ]
Replace with 'i8 6':   %split.ph.us = phi i8 [ 6, %for.body.us ]        
Replace with 'i8 6':   %split = phi i8 [ undef, %for.cond.for.end_crit_edge.us-lcssa ], [ 6, %for.cond.for.end_crit_edge.us-lcssa.us ]                                               
Replace with 'i8 6':   %cj.sroa.0.0.lcssa = phi i8 [ 6, %for.cond.for.end_crit_edge ], [ undef, %entry ]                                                                             
Replace with 'i32 6':   %conv4 = sext i8 6 to i32

[davide]

lib/Transforms/Scalar/LoopUnswitch.cpp

1278–1279 ↗

(On Diff #97174)

Alternatively this could use isInstructionTriviallyDead. At that point LoopUnswitch should require TLI I guess and we should thread it properly in the pass.
I don't mind doing that, FWIW, if you prefer and/or it's better.

[davide]

See https://bugs.llvm.org/show_bug.cgi?id=32818 for the original testcase.

[davide]

lib/Transforms/Scalar/LoopUnswitch.cpp

1278–1279 ↗

(On Diff #97174)

Nevermind, I was wrong, it doesn't require TLI.

[chandlerc]

This seems correct and safe. Happy for you to land as-is, or to try a different approach if you want. I don't think you're making anything worse, that's for sure. =D

lib/Transforms/Scalar/LoopUnswitch.cpp

1278–1279 ↗

(On Diff #97174)

FWIW, I would keep this surgical given the work I'm doing on LoopUnswitch. But I don't feel strongly either way.

llvm/trunk/lib/Transforms/Scalar/LoopUnswitch.cpp

if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
       Worklist.push_back(Use);
 
   // Add users to the worklist which may be simplified now.
   for (User *U : I->users())
     Worklist.push_back(cast<Instruction>(U));
   LPM->deleteSimpleAnalysisValue(I, L);
   RemoveFromWorklist(I, Worklist);
   I->replaceAllUsesWith(V);
+  if (!I->mayHaveSideEffects())
     I->eraseFromParent();
   ++NumSimplify;
 }
 
 /// We know either that the value LIC has the value specified by Val in the
 /// specified loop, or we know it does NOT have that value.
 /// Rewrite any uses of LIC or of properties correlated to it.
 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
                                                         Constant *Val,

llvm/trunk/test/Transforms/LoopUnswitch/pr32818.ll

+; Check that the call doesn't get removed even if
+; it has no uses. It could have side-effects.
+; RUN: opt -loop-unswitch -S %s | FileCheck %s
+
+; CHECK-LABEL: @tinky
+define i32 @tinkywinky(i8 %patatino) {
+  %cmp1 = icmp slt i8 %patatino, 5
+  br label %body
+body:
+  %i = select i1 %cmp1, i8 6, i8 undef
+  br i1 true, label %body, label %end
+end:
+  %split = phi i8 [ %i, %body ]
+  %conv4 = sext i8 %split to i32
+; CHECK: tail call fastcc i32 @fn5(
+  %call = tail call fastcc i32 @fn5(i32 %conv4)
+  ret i32 0
+}
+declare fastcc i32 @fn5(i32 returned) unnamed_addr