[LLVMdev] GVN miscompile debugging help

[Villmow, Micah]

Ok, after discussing with this internally some more because of this email, its determined that GVN is doing the right thing here, so this email can be ignored.
Micah

From: Villmow, Micah
Sent: Friday, August 10, 2012 12:49 PM
To: Developers Mailing List
Cc: 'Duncan Sands'
Subject: RE: GVN miscompile debugging help

Ok, I think I have a handle on the problem. The changes that cause this were introduced in R141177.

Take this basic block:
f.end:                                           ; preds = %if.else, %if.then62, %if.then
  %x.0 = phi i32 [ %conv, %if.then ], [ %tmp81, %if.then62 ], [ %tmp55, %if.else ]
  %y.0 = phi i32 [ %conv37, %if.then ], [ %tmp71, %if.then62 ], [ %conv45, %if.else ]
  ...
  %cmp106 = icmp eq i32 %x.0, %y.0
  br i1 %cmp106, label %if.then108, label %if.else109

When processing the branch instruction, GVN somehow thinks that it is safe to switch %y.0 for %x.0

Here is if.then108:
if.then108:                                       ; preds = %if.end
  br i1 %cmp83, label %if.then113, label %if.end112

Here is if.end112:
if.end112:                                        ; preds = %if.then113, %if.then108
  tail call  void @barrier(i32 0, i32 1) nounwind
  %cmp151 = icmp ult i32 %tmp95, 216
  %tmp153 = shl i32 %x.0, 5
  %tmp155 = or i32 %., %tmp153
  %tmp209 = extractelement <4 x float> %tmp99, i32 0
  %tmp210 = insertelement <3 x float> undef, float %tmp209, i32 0
  %tmp211 = extractelement <4 x float> %tmp99, i32 1
  %tmp212 = insertelement <3 x float> %tmp210, float %tmp211, i32 1
  %tmp213 = extractelement <4 x float> %tmp99, i32 2
  %tmp214 = insertelement <3 x float> %tmp212, float %tmp213, i32 2
  %tmp279 = extractelement <4 x float> %tmp99, i32 3
  %tmp280 = fmul float %tmp279, 0xC061252760000000
  br label %for.body

Somehow, this optimization on the br instruction is turning

  %tmp153 = shl i32 %y.0, 5
into
  %tmp153 = shl i32 %x.0, 5

breaking our applications.

My proposed solution is to add this to propagateEquality:
  // Don't try to propogate equalities between phi nodes.
    if (isa<PHINode>(LHS) || isa<PHINode>(RHS)) continue;

The only other thing I can think of is that isOnlyReachableViaThisEdge has a bug in its logic in this case.

Micah


From: llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu> [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Villmow, Micah
Sent: Friday, August 10, 2012 11:34 AM
To: Developers Mailing List
Subject: [LLVMdev] GVN miscompile debugging help

I found a case where GVN seems to miscompile an OpenCL program. What I am trying to figure out is given a bitcode file, how can I reduce it to a simpler case with bugpoint when I don't have a valid reference compiler available.

Thanks for any tips,
Micah
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