[llvm] fc93780 - Don't jump to landing pads in Control Flow Optimizer

[Tom Stellard via llvm-commits]

On 05/26/2020 10:13 AM, Arthur Eubanks wrote:
> Hi Tom, there's a request to merge this bug fix into 10.0.1, is that ok?
> 

I've added this to my queue for review.

-Tom
> On Thu, May 21, 2020 at 3:19 PM Arthur Eubanks via llvm-commits <llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org>> wrote:
> 
> 
>     Author: Arthur Eubanks
>     Date: 2020-05-21T15:19:10-07:00
>     New Revision: fc937806efd71eb3803b35d0920bb7d76bc3040b
> 
>     URL: https://github.com/llvm/llvm-project/commit/fc937806efd71eb3803b35d0920bb7d76bc3040b
>     DIFF: https://github.com/llvm/llvm-project/commit/fc937806efd71eb3803b35d0920bb7d76bc3040b.diff
> 
>     LOG: Don't jump to landing pads in Control Flow Optimizer
> 
>     Summary: Likely fixes https://bugs.llvm.org/show_bug.cgi?id=45858.
> 
>     Subscribers: hiraditya, llvm-commits
> 
>     Tags: #llvm
> 
>     Differential Revision: https://reviews.llvm.org/D80047
> 
>     Added:
>         llvm/test/CodeGen/X86/branchfolding-ehpad.mir
> 
>     Modified:
>         llvm/lib/CodeGen/BranchFolding.cpp
> 
>     Removed:
> 
> 
> 
>     ################################################################################
>     diff  --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
>     index eb72542b6efe..852bfb36c720 100644
>     --- a/llvm/lib/CodeGen/BranchFolding.cpp
>     +++ b/llvm/lib/CodeGen/BranchFolding.cpp
>     @@ -921,10 +921,10 @@ bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
>            continue;
>          }
> 
>     -    // If one of the blocks is the entire common tail (and not the entry
>     -    // block, which we can't jump to), we can treat all blocks with this same
>     -    // tail at once.  Use PredBB if that is one of the possibilities, as that
>     -    // will not introduce any extra branches.
>     +    // If one of the blocks is the entire common tail (and is not the entry
>     +    // block/an EH pad, which we can't jump to), we can treat all blocks with
>     +    // this same tail at once.  Use PredBB if that is one of the possibilities,
>     +    // as that will not introduce any extra branches.
>          MachineBasicBlock *EntryBB =
>              &MergePotentials.front().getBlock()->getParent()->front();
>          unsigned commonTailIndex = SameTails.size();
>     @@ -932,19 +932,21 @@ bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
>          // into the other.
>          if (SameTails.size() == 2 &&
>              SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
>     -        SameTails[1].tailIsWholeBlock())
>     +        SameTails[1].tailIsWholeBlock() && !SameTails[1].getBlock()->isEHPad())
>            commonTailIndex = 1;
>          else if (SameTails.size() == 2 &&
>                   SameTails[1].getBlock()->isLayoutSuccessor(
>     -                                                     SameTails[0].getBlock()) &&
>     -             SameTails[0].tailIsWholeBlock())
>     +                 SameTails[0].getBlock()) &&
>     +             SameTails[0].tailIsWholeBlock() &&
>     +             !SameTails[0].getBlock()->isEHPad())
>            commonTailIndex = 0;
>          else {
>            // Otherwise just pick one, favoring the fall-through predecessor if
>            // there is one.
>            for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
>              MachineBasicBlock *MBB = SameTails[i].getBlock();
>     -        if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
>     +        if ((MBB == EntryBB || MBB->isEHPad()) &&
>     +            SameTails[i].tailIsWholeBlock())
>                continue;
>              if (MBB == PredBB) {
>                commonTailIndex = i;
> 
>     diff  --git a/llvm/test/CodeGen/X86/branchfolding-ehpad.mir b/llvm/test/CodeGen/X86/branchfolding-ehpad.mir
>     new file mode 100644
>     index 000000000000..d445cd206803
>     --- /dev/null
>     +++ b/llvm/test/CodeGen/X86/branchfolding-ehpad.mir
>     @@ -0,0 +1,28 @@
>     +# RUN: llc -mtriple=x86_64-windows-msvc -verify-machineinstrs -run-pass branch-folder -o - %s | FileCheck %s
>     +
>     +# Check that branch-folder does not create a fallthrough to a landing pad.
>     +# Also make sure that the landing pad still can be tail merged.
>     +---
>     +name: foo
>     +body: |
>     +  ; CHECK-LABEL: name: foo
>     +  bb.0:
>     +    successors: %bb.1, %bb.3
>     +  bb.1:
>     +    JCC_1 %bb.4, 5, implicit killed $eflags
>     +  bb.2:
>     +    MOV8mi $r13, 1, $noreg, 0, $noreg, 0
>     +    JMP_1 %bb.5
>     +  ; CHECK: bb.2:
>     +  ; CHECK-NOT: successors: {{.*}}bb.3
>     +  ; CHECK: bb.3 (landing-pad):
>     +  ; CHECK-NOT: MOV8mi
>     +  bb.3(landing-pad):
>     +    MOV8mi $r13, 1, $noreg, 0, $noreg, 0
>     +    JMP_1 %bb.5
>     +  ; CHECK: bb.4:
>     +  bb.4:
>     +    MOV8mi $r13, 2, $noreg, 0, $noreg, 0
>     +  bb.5:
>     +    RET 0
>     +...
> 
> 
> 
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