https://bugs.llvm.org/show_bug.cgi?id=49913
Bug ID: 49913
Summary: [coroutines] errno address is reused after context
switch
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: C++2a
Assignee: unassignedclangb...@nondot.org
Reporter: antosh...@gmail.com
CC: blitzrak...@gmail.com, erik.pilking...@gmail.com,
llvm-bugs@lists.llvm.org, richard-l...@metafoo.co.uk
Created attachment 24742
--> https://bugs.llvm.org/attachment.cgi?id=24742&action=edit
Full example
Consider the example:
CoroTask CoroToDealWith() {
if (errno) first_call();
co_await writerQueue;
if (errno) second_call();
}
With `-O2 -std=c++20 -stdlib=libc++` flags clang generates code that stores the
`errno` address on first call:
call __errno_location
mov qword ptr [rbx + 24], rax
cmp dword ptr [rax], 0
je .LBB1_2
call first_call()
and reuses that address after wakeup:
CoroToDealWith() [clone .resume]: # @CoroToDealWith() [clone .resume]
push rbx
mov rbx, rdi
mov rax, qword ptr [rdi + 24]
cmp dword ptr [rax], 0
je .LBB2_2
call second_call()
The optimization is wrong: coroutine could be resumed on other thread and the
errno address changes.
Godbolt playground: https://godbolt.org/z/Wh9xd9oME
Looks like the common subexpression elimination pass must not be applied
between the context switches for the functions with __attribute__((const))
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