[Bug c++/89937] New: For example code, which is valid as either C or C++, optimization seems much better for C

[wkaras at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89937

            Bug ID: 89937
           Summary: For example code, which is valid as either C or C++,
                    optimization seems much better for C
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wkaras at yahoo dot com
  Target Milestone: ---

I compiled this code with -O2, and with either -x c or -x c++ .  The
optimization seems to work much better for C than for C++.

typedef struct TSFastDbgCntl_
{
    const char * const tag; // nul-terminated string
    const char * const on;  // pointer to 1-byte flag
}
TSFastDbgCntl;

TSFastDbgCntl * TSCreateFastDbgCntl(const char *tag);

#include <stdarg.h>
void TSVDebug(const char *tag, const char *fmt, va_list args);
inline void TSFastDbg(TSFastDbgCntl *fd_cntl, const char *fmt, ...)
{
    if (fd_cntl->on)
    {
        va_list ap;
        va_start(ap, fmt);
        TSVDebug(fd_cntl->tag, fmt, ap);
    }
}

void dummy(int i, double d)
{
    TSFastDbgCntl *fd_cntl = TSCreateFastDbgCntl("pluggymcplugin");
    TSFastDbg(fd_cntl, "Test %d %f", i * 5, d * 7);
    TSFastDbg(fd_cntl, "Test fixed string");
}

https://godbolt.org/z/d2MzPo

Note that the difference in optimization of this code for C vs. C++ is similar
for clang/LLVM.