https://bugs.llvm.org/show_bug.cgi?id=49403
Bug ID: 49403
Summary: branches containing only llvm.assume are not fully
eliminated
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Common Code Generator Code
Assignee: ja...@google.com
Reporter: ja...@google.com
CC: llvm-bugs@lists.llvm.org
[patch in progress]
When a branch contains only a call to `llvm.assume`, the generated machine code
still contains the condition of that branch, even though the branch itself and
its body are gone.
The issue seems to be that the call to `llvm.assume` is only removed very late,
in the "CodeGen Prepare (codegenprepare)" pass; the resulting empty branch
survives the move into MIR and only disappears in the MIR pass "Control Flow
Optimizer (branch-folder)".
Reproducer:
```
define dso_local void @a(i64 %addr, i64 %addr2) {
entry:
%cmp = icmp ne i64 %addr, %addr2
%cmp1 = icmp eq i64 %addr, 0
br i1 %cmp1, label %do_assume, label %after_assume
do_assume:
tail call void @llvm.assume(i1 %cmp)
br label %after_assume
after_assume:
br i1 %cmp, label %end, label %call_b
call_b:
tail call void @b(i64 %addr)
br label %end
end:
ret void
}
declare void @llvm.assume(i1 noundef)
declare void @b(i64)
```
Resulting X86 assembly:
```
# %bb.0: # %entry
testq %rdi, %rdi
cmpq %rsi, %rdi
je .LBB0_2
# %bb.1: # %end
retq
.LBB0_2: # %call_b
jmp b@PLT # TAILCALL
```
If you remove the call to @llvm.assume from the testcase, the entire branch
including its condition disappears in the codegenprepare pass.
So the issue is probably that after CodeGenPrepare::optimizeCallInst() may have
killed the call instruction, we don't try to eliminate the block with
CodeGenPrepare::eliminateMostlyEmptyBlocks()? And this is the case because
CodeGenPrepare::runOnFunction() first calls eliminateMostlyEmptyBlocks(), then
runs the loop that calls optimizeBlock().
I'll try to send a patch for this in a bit.
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