[cfe-dev] Clang optimizing an infinite loop

[Gaier, Bjoern via cfe-dev]

Hello everyone,

for testing our code I wanted to create an infinite loop with Clang, so I wrote the most basic infinite loop I came up with:
void Sample()
{
                while(true);
}

To my surprise Clang generated the following assembly code:
                .def       "?Sample@@YAXXZ";
                .scl         2;
                .type     32;
                .endef
                .section .text,"xr",one_only,"?Sample@@YAXXZ"
                .globl     "?Sample@@YAXXZ"            # -- Begin function ?Sample@@YAXXZ
                .p2align                4, 0x90
"?Sample@@YAXXZ":                   # @"?Sample@@YAXXZ"
.Lfunc_begin4:
                .cv_func_id 6
# %bb.0:
                nop                                     # avoids zero-length function
.Lfunc_end4:
                                        # -- End function

Which ended in an access violation D:
Why was Clang doing that? Is it because of some loop optimization since the body of the loop is empty and nothing else happens in the loop? :0
But why was it missing the "ret" instruction then?

The full command line without include directories is:
1>clang version 13.0.0 (https://github.com/llvm/llvm-project.git 5f500d73cd1aaff4c9ab2fd5c327c2d5ca9ae5c9)
1>Target: x86_64-pc-windows-msvc
1>Thread model: posix
1> "clang-cl.exe" -cc1 -triple x86_64-pc-windows-msvc19.28.29912 -S -disable-free -disable-llvm-verifier -discard-value-names -main-file-name CMException.cpp -mrelocation-model pic -pic-level 2 -mframe-pointer=none -relaxed-aliasing -fmath-errno -fno-rounding-math -mconstructor-aliases -munwind-tables -target-cpu skylake -target-feature +fsgsbase -mllvm -x86-asm-syntax=intel -mllvm -treat-scalable-fixed-error-as-warning -D_MT -flto-visibility-public-std --dependent-lib=libcmt --dependent-lib=oldnames -stack-protector 2 -fcxx-exceptions -fexceptions -fexternc-nounwind -fms-volatile -fdefault-calling-conv=cdecl -fdiagnostics-format msvc -gno-column-info -gcodeview -debug-info-kind=limited -v -ffunction-sections -D WIN32 -D NTSYSAD_CM -D CODEMODULE -D _CONSOLE -D _UNICODE -D NDEBUG -D _CRT_SECURE_NO_WARNINGS -D _CRT_USE_BUILTIN_OFFSETOF -D NOMINMAX -D _UNICODE -D UNICODE -O2 -Wall -Wno-error -Wno-unused-command-line-argument -Wno-microsoft-cast -Wno-writable-strings -Wno-microsoft-enum-forward-reference -Wno-invalid-token-paste -fdeprecated-macro -ferror-limit 3000 -fno-use-cxa-atexit -fms-extensions -fms-compatibility -fms-compatibility-version=19.28.29912 -std=c++17 -fdelayed-template-parsing -fno-caret-diagnostics -vectorize-loops -vectorize-slp -faddrsig -o "x64\\Release\\CMException.asm" -x c++ CMException.cpp

Thank you for any help in advance!

Kind regards
Björn
Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816, USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus Bode, Takashi Nagano, Tetsuhiro Habe, Ergin Cansiz.
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