[LLVMbugs] [Bug 23993] New: clang-cl /arch:IA32 generates CMOV instructions not supported by selected arch

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23993

            Bug ID: 23993
           Summary: clang-cl /arch:IA32 generates CMOV instructions not
                    supported by selected arch
           Product: clang
           Version: 3.6
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: amadvance at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Using clang-cl /arch:IA32 should not generate CMOV instructions that are not
supported by a generic i686.

Visual Studio documentation is clear about that. CMOV instructions should be
generated only with /arch:SSE2.

See: https://msdn.microsoft.com/en-us/library/7t5yh4fd.aspx
"In addition to using the SSE and SSE2 instructions, the compiler also uses
other instructions that are present on the processor revisions that support SSE
and SSE2. An example is the CMOV instruction that first appeared on the Pentium
Pro revision of the Intel processors."

The problem seems to be the "-target-cpu pentium4" selection. A "i686" would
work.

A workaround is to add in the command line: -Xclang -target-cpu -Xclang i686

Here an example with clang 3.6.0:

...> type test.c
extern int i;
extern int j;
int f(int g) {
  i++;
  if (g)
    return j;
  else
    return 1;
}

...> clang-cl -O2 /arch:IA32 -v -c test.c -o test.obj
clang version 3.6.0 (tags/RELEASE_360/final)
Target: i686-pc-windows-msvc
Thread model: posix
 "D:\\compiler\\clang-3.6\\bin\\clang-cl.exe" -cc1 -triple i686-pc-windows-msvc
-emit-obj -disable-free -main-file-name
test.c -mrelocation-model static -mthread-model posix -mdisable-fp-elim
-relaxed-aliasing -fmath-errno -masm-verbose -mc
onstructor-aliases -target-cpu pentium4 -D_MT --dependent-lib=libcmt
--dependent-lib=oldnames -fdiagnostics-format msvc
-v -dwarf-column-info -coverage-file
"C:\\Users\\am\\Desktop\\clangtest\\test.c" -resource-dir
"D:\\compiler\\clang-3.6\
\bin\\..\\lib\\clang\\3.6.0" -internal-isystem
"D:\\compiler\\clang-3.6\\bin\\..\\lib\\clang\\3.6.0\\include" -internal-
isystem "d:\\compiler\\msvc71\\vc7\\include" -internal-isystem
"d:\\compiler\\msvc71\\vc7\\atlmfc\\include" -internal-is
ystem "d:\\compiler\\msvc71\\vc7\\PlatformSDK\\include" -O2
-fdebug-compilation-dir "C:\\Users\\am\\Desktop\\clangtest"
-ferror-limit 19 -fmessage-length 120 -mstackrealign -fms-extensions
-fms-compatibility -fms-compatibility-version=17.00
 -fdelayed-template-parsing -fobjc-runtime=gcc -fdiagnostics-show-option
-fcolor-diagnostics -vectorize-loops -vectorize
-slp -o test.obj -x c test.c
clang -cc1 version 3.6.0 based upon LLVM 3.6.0 default target
i686-pc-windows-gnu
#include "..." search starts here:
#include <...> search starts here:
 D:\compiler\clang-3.6\bin\..\lib\clang\3.6.0\include
 d:\compiler\msvc71\vc7\include
 d:\compiler\msvc71\vc7\atlmfc\include
 d:\compiler\msvc71\vc7\PlatformSDK\include
End of search list.

...> dumpbin /DISASM test.obj
Dump of file test.obj

File Type: COFF OBJECT

_f:
  00000000: 55                 push        ebp
  00000001: 89 E5              mov         ebp,esp
  00000003: FF 05 00 00 00 00  inc         dword ptr [_i]
  00000009: 83 7D 08 00        cmp         dword ptr [ebp+8],0
  0000000D: B8 01 00 00 00     mov         eax,1
  00000012: 0F 45 05 00 00 00  cmovne      eax,dword ptr [_j]
            00
  00000019: 5D                 pop         ebp
  0000001A: C3                 ret

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