optimization/7226: Spurious "output number 1 not directly addressable" error with asm =m constraint

[Trevor Blackwell <tlb at tlb dot blackwells dot net>]

>Number:         7226
>Category:       optimization
>Synopsis:       gcc -O can't compile some asm functions with memory operands
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          ice-on-legal-code
>Submitter-Id:   net
>Arrival-Date:   Sat Jul 06 17:46:02 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Trevor Blackwell
>Release:        3.1.1 20020617 (prerelease) [FreeBSD]
>Organization:
>Environment:
System: FreeBSD tlb.blackwells.net 4.5-RELEASE-p2 FreeBSD 4.5-RELEASE-p2 #3: Sat Mar 9 12:32:15 PST 2002 root@tlb.umbrellaresearch.com:/d1/usr-obj/usr/src/sys/TLB i386


	
host: i386-portbld-freebsd4.5
build: i386-portbld-freebsd4.5
target: i386-portbld-freebsd4.5
configured with: ./..//gcc-20020617/configure --disable-nls --with-gnu-as --with-gnu-ld --with-gxx-include-dir=/usr/local/lib/gcc-lib/i386-portbld-freebsd4.5/3.1.1/include/g++ --disable-libgcj --disable-shared --prefix=/usr/local i386-portbld-freebsd4.5
>Description:

When I compile, I get:
$ g++31 -O -o t-gcc-asm t-gcc-asm.cc
t-gcc-asm.cc: In function `int main()':
t-gcc-asm.cc:13: output number 1 not directly addressable
  
The asm function below should set foo=13 and bar=14.

It works if you compile with -O0, or if you declare bar volatile. 
It works if you add a (pointless) statement &bar; ahead of the asm,
to force bar into memory.
It works if you make it be a C program instead of C++.

gcc 2.95.3 compiles it OK, either as C++ or C.

Presumably the =m constraint isn't forcing the operand to memory like
it should.

I don't see the same problem with only 1 "=m" operand.

>How-To-Repeat:

Compile this C++ program with -O. Add -DFIX1 or -DFIX2 for workarounds.

---- t-gcc-asm.cc ---

extern "C" int printf(const char *, ...);

int main()
{
  int foo;

#ifdef FIX1
  volatile int bar;
#else
  int bar;
#endif
  
#ifdef FIX2
  &bar;
#endif

  asm("push %%eax;"
      "mov $13, %%eax;"
      "mov %%eax, %0;"
      "mov $14, %%eax;"
      "mov %%eax, %1;"
      "pop %%eax;"
      : "=m" (foo), "=m" (bar));

  printf("foo=%d bar=%d\n",foo,bar);
}

-----------------------

>Fix:

- Declare bar volatile. 

- Use a register constraint (but this doesn't work in my application, because 
it's already using all available registers.)

- Take the address of bar somewhere in the function.

>Release-Note:
>Audit-Trail:
>Unformatted: