From: utzoo!decvax!harpo!eagle!mit-vax!mp
Newsgroups: net.lang.c,net.unix-wizards
Title: pcc register allocation bug
Article-I.D.: mit-vax.216
Posted: Mon Apr  4 18:41:04 1983
Received: Tue Apr  5 04:17:46 1983

Awhile ago I passed on a problem that was observed in the
4.1bsd (and System III and V) pcc.  It went like this:

    From: Jeff Mogul <mogul at Shasta>

    The problem arises whenever an expression includes a comparison
    of a float and a double, and at least one other operation.  Either
    the expression doesn't evaluate correctly because a temporary register
    is being used for two different things, or it does evaluate correctly,
    but a live register may be bashed.
    
    Here is a program that fails:
    
    #include	<stdio.h>
    
    double	dd[] =	{ 0.0, 2.0 };
    float	ff[] =	{ 0.0, 1.0 };
    int i = 1;
    
    main(){
	    if( ff[i] >= dd[i] ) printf( "ff >= dd\n" );
    }
    
    
    Here is the buggy Vax machine code (cc -S, no optimizer):
    
	    movl	_i,r0
	    movl	_i,r1
	    cvtfd	_ff[r0],r0
	    cmpd	r0,_dd[r1]
	    jlss	L25
    
    
Jeff Mogul and John Gill, of Stanford, found a way to fix the bug.
They write:

    It involves changing one file (/usr/[local/]src/cmd/pcc/table.c),
    recompiling, and re-installing (using "make", of course.)
    
    *** /usr/src/cmd/pcc/table.c	Wed Dec 17 15:04:41 1980
    --- ntable.c	Sun Mar  6 11:00:36 1983
    ***************
    *** 228,234
      OPLOG,	FORCC,
      	SAREG|AWD,	TDOUBLE,
      	SAREG|AWD,	TFLOAT,
    ! 		NAREG|NASR,	RESCC,
      		"	cvtfd	AR,A1\n	cmpd	AL,A1\nZP",
      
      OPLOG,	FORCC,
    
    --- 228,234 -----
      OPLOG,	FORCC,
      	SAREG|AWD,	TDOUBLE,
      	SAREG|AWD,	TFLOAT,
    ! 		2*NTEMP,	RESCC,
      		"	cvtfd	AR,A1\n	cmpd	AL,A1\nZP",
      
      OPLOG,	FORCC,
    ***************
    *** 234,240
      OPLOG,	FORCC,
      	SAREG|AWD,	TFLOAT,
      	SAREG|AWD,	TDOUBLE,
    ! 		NAREG|NASL,	RESCC,
      		"	cvtfd	AL,A1\n	cmpd	A1,AR\nZP",
      
      OPLOG,	FORCC,
    
    --- 234,240 -----
      OPLOG,	FORCC,
      	SAREG|AWD,	TFLOAT,
      	SAREG|AWD,	TDOUBLE,
    ! 		2*NTEMP,	RESCC,
      		"	cvtfd	AL,A1\n	cmpd	A1,AR\nZP",
      
      OPLOG,	FORCC,