[cfe-dev] Optimization creates infinite loop

[Gaier, Bjoern via cfe-dev]

Hello Clang-Developer,

We ran into a - for us big problem - with the Clang compiler. We had the following code:
extern double RT_Time;
void Sample1sec()
{
       const double startTime = RT_Time;
       while(RT_Time < (startTime + 5.0005));
}

The value of RT_Time will change after a while and the loop is supposed to wait for it. However the assembly code will lead into an endless loop if the condition is not meet the first time:
"?Sample1sec@@YAXXZ":                   # @"?Sample1sec@@YAXXZ"
.Lfunc_begin0:
                .cv_func_id 0
# %bb.0:
                .cv_file 1 "C:\\Users\\h16020\\Desktop\\Projects\\Squirrel\\Project\\Leviathan\\CM_Driver1\\CM_Driver1.cpp" "4E92768969662F6E513C11595FEA9D63" 1
                .cv_loc  0 1 5 0                 # CM_Driver1.cpp:5:0
                movsd  xmm0, qword ptr [rip + "?RT_Time@@3NA"] # xmm0 = mem[0],zero
.Ltmp0:
                #DEBUG_VALUE: Sample1sec:startTime <- $xmm0
                movsd  xmm1, qword ptr [rip + __real at 40140083126e978d] # xmm1 = mem[0],zero
                .cv_loc  0 1 0 0                 # CM_Driver1.cpp:0:0
                addsd   xmm1, xmm0
                ucomisd               xmm1, xmm0
                .cv_loc  0 1 6 0                 # CM_Driver1.cpp:6:0
                jbe         .LBB0_2
.Ltmp1:
                .p2align                4, 0x90
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
                #DEBUG_VALUE: Sample1sec:startTime <- $xmm0
                jmp        .LBB0_1
.Ltmp2:
.LBB0_2:
                #DEBUG_VALUE: Sample1sec:startTime <- $xmm0
                .cv_loc  0 1 7 0                 # CM_Driver1.cpp:7:0
                ret
.Ltmp3:
.Lfunc_end0:

Marking RT_Time with volatile fixes the problem. But I wonder, why is doing Clang that optimization? RT_Time was not declared 'const' so didn't Clang had to know the value can change?
Is this a bug or do I understand something wrong?

Kind greetings and thank you for the help in advance,
Björn

P.S.
My compile flags (when using -###) are:
clang-cl.exe "-cc1" "-triple" "x86_64-pc-windows-msvc19.16.27034" "-S" "-disable-free" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "CM_Driver1.cpp" "-mrelocation-model" "pic" "-pic-level" "2" "-mthread-model" "posix" "-relaxed-aliasing" "-fmath-errno" "-masm-verbose" "-mconstructor-aliases" "-munwind-tables" "-target-cpu" "x86-64" "-mllvm" "-x86-asm-syntax=intel" "-D_MT" "-flto-visibility-public-std" "--dependent-lib=libcmt" "--dependent-lib=oldnames" "-fcxx-exceptions" "-fexceptions" "-fexternc-nounwind" "-fms-volatile" "-fdefault-calling-conv=cdecl" "-fdiagnostics-format" "msvc" "-gcodeview" "-debug-info-kind=limited" "-momit-leaf-frame-pointer" "-ffunction-sections" "-coverage-notes-file" "C:\\Users\\h16020\\Desktop\\Projects\\Squirrel\\Project\\Leviathan\\CM_Driver1\\CM_Driver1.gcno" "-resource-dir" "S:\\STSHARE\\EXTERNAL\\LLVM\\lib\\clang\\9.0.0" "-D" "WIN32" "-D" "NTSYSAD_CM" "-D" "_CONSOLE" "-D" "CODEMODULE" "-D" "__asm=RemoveAssembly" "-D" "_UNICODE" "-D" "UNICODE" "-internal-isystem" "S:\\STSHARE\\EXTERNAL\\LLVM\\lib\\clang\\9.0.0\\include" "-internal-isystem" "S:\\STSHARE\\EXTERNAL\\LLVM\\Include" "-internal-isystem" "S:\\STEXEC\\ExecutionEnvironment\\INCLUDE" "-internal-isystem" "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\Professional\\VC\\Tools\\MSVC\\14.16.27023\\include" "-internal-isystem" "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\Professional\\VC\\Tools\\MSVC\\14.16.27023\\atlmfc\\include" "-internal-isystem" "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\Professional\\VC\\Auxiliary\\VS\\include" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.10240.0\\ucrt" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\8.1\\Include\\um" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\8.1\\Include\\shared" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\8.1\\Include\\winrt" "-internal-isystem" "C:\\Program Files (x86)\\Windows Kits\\NETFXSDK\\4.6.1\\Include\\um" "-O2" "-WCL4" "-Wno-error" "-Wno-unused-command-line-argument" "-Wno-microsoft-cast" "-Wno-writable-strings" "-Wno-microsoft-enum-forward-reference" "-Wno-invalid-token-paste" "-fdeprecated-macro" "-fdebug-compilation-dir" "C:\\Users\\h16020\\Desktop\\Projects\\Squirrel\\Project\\Leviathan\\CM_Driver1" "-ferror-limit" "3000" "-fmessage-length" "0" "-fno-use-cxa-atexit" "-fms-extensions" "-fms-compatibility" "-fms-compatibility-version=19.16.27034" "-std=c++14" "-fno-threadsafe-statics" "-fdelayed-template-parsing" "-finline-functions" "-fobjc-runtime=gcc" "-fno-caret-diagnostics" "-fdiagnostics-show-option" "-fno-show-column" "-vectorize-loops" "-vectorize-slp" "-faddrsig" "-o" "x64\\Release\\CM_Driver1.asm" "-x" "c++" "CM_Driver1.cpp"

clang-cl.exe "-cc1as" "-triple" "x86_64-pc-windows-msvc19.16.27034" "-filetype" "obj" "-main-file-name" "CM_Driver1.cpp" "-target-cpu" "x86-64" "-dwarf-version=4" "-mrelocation-model" "pic" "-mincremental-linker-compatible" "-o" "x64\\Release\\CM_Driver1.obj" "x64\\Release\\CM_Driver1.asm"
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