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: unassignedb...@nondot.org
Reporter: ndesaulni...@google.com
CC: chandl...@gmail.com, craig.top...@gmail.com,
h...@chromium.org, jhaber...@gmail.com,
llvm-bugs@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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