[llvm-bugs] [Bug 50130] New: conditional branch to unconditional tailcall not folded to conditional tailcall

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50130

            Bug ID: 50130
           Summary: conditional branch to unconditional tailcall not
                    folded to conditional tailcall
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: ndesaulniers at google.com
                CC: chandlerc at gmail.com, craig.topper at gmail.com,
                    hans at chromium.org, jhaberman at gmail.com,
                    llvm-bugs at lists.llvm.org

The blog post
https://blog.reverberate.org/2021/04/21/musttail-efficient-interpreters.html
and comment thread https://news.ycombinator.com/item?id=26934616#26937585 point
to a test case (reduced):

```
target triple = "x86_64-unknown-linux-gnu"

declare dso_local i8* @foo(i8*, i64)
declare dso_local i8* @bar(i8*, i64)

define i8* @baz(i8* %ptr, i64 %data) {
; CHECK-LABEL: baz:
; CHECK:       # %bb.0: # %entry
; CHECK-NEXT:    testb %sil, %sil
; CHECK-NEXT:    je foo # TAILCALL
; CHECK-NEXT:  # %bb.1: # %if.end
; CHECK-NEXT:    addq $5, %rdi
; CHECK-NEXT:    jmp bar # TAILCALL
entry:
  %and = and i64 %data, 255
  %tobool.not = icmp eq i64 %and, 0
  br i1 %tobool.not, label %if.end, label %if.then

if.then:                                          ; preds = %entry
  %call = musttail call i8* @foo(i8* %ptr, i64 %data)
  ret i8* %call

if.end:                                           ; preds = %entry
  %add.ptr4 = getelementptr inbounds i8, i8* %ptr, i64 5
  %call5 = musttail call i8* @bar(i8* nonnull %add.ptr4, i64 %data)
  ret i8* %call5
}
```
run through llc will instead produce:
```
baz:
  # %bb.0:
  testb   %sil, %sil
  je      .LBB0_2
  # %bb.1: # %if.then
  jmp     foo
.LBB0_2: # %if.end
  addq    $5, %rdi
  jmp     bar
```
though we could have produced:
```
baz:
  # %bb.0: # %entry
  testb %sil, %sil
  je foo # TAILCALL
  # %bb.1: # %if.end
  addq $5, %rdi
  jmp bar # TAILCALL
```

Some code added in D29856 in llvm/lib/CodeGen/BranchFolding.cpp looks like it
could do the folding.  Quick experimentation with removing the guards:
1. OptForSize
2. PredTBB == MBB

enables this optimizations, but seems to regress quite a few tests:
Failed Tests (29):
  LLVM :: CodeGen/X86/add.ll
  LLVM :: CodeGen/X86/atom-pad-short-functions.ll
  LLVM :: CodeGen/X86/avx512-i1test.ll
  LLVM :: CodeGen/X86/bmi.ll
  LLVM :: CodeGen/X86/brcond.ll
  LLVM :: CodeGen/X86/btq.ll
  LLVM :: CodeGen/X86/cmp.ll
  LLVM :: CodeGen/X86/conditional-tailcall-pgso.ll
  LLVM :: CodeGen/X86/conditional-tailcall.ll
  LLVM :: CodeGen/X86/copy-eflags.ll
  LLVM :: CodeGen/X86/extern_weak.ll
  LLVM :: CodeGen/X86/fold-rmw-ops.ll
  LLVM :: CodeGen/X86/fp-strict-scalar-cmp.ll
  LLVM :: CodeGen/X86/funnel-shift.ll
  LLVM :: CodeGen/X86/neg_cmp.ll
  LLVM :: CodeGen/X86/or-branch.ll
  LLVM :: CodeGen/X86/peep-test-4.ll
  LLVM :: CodeGen/X86/pr37063.ll
  LLVM :: CodeGen/X86/rd-mod-wr-eflags.ll
  LLVM :: CodeGen/X86/sibcall.ll
  LLVM :: CodeGen/X86/slow-incdec.ll
  LLVM :: CodeGen/X86/sqrt-partial.ll
  LLVM :: CodeGen/X86/switch-bt.ll
  LLVM :: CodeGen/X86/tail-call-conditional.mir
  LLVM :: CodeGen/X86/tail-opts.ll
  LLVM :: CodeGen/X86/tailcall-cgp-dup.ll
  LLVM :: CodeGen/X86/tailcall-extract.ll
  LLVM :: CodeGen/X86/xor-icmp.ll
  LLVM :: DebugInfo/COFF/pgo.ll

Some of these are straightforward fixes that I think make sense, but others
like llvm/test/CodeGen/X86/conditional-tailcall.ll look quite wrong (branching
the wrong way, IIUC)!

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