Re: Propagation of pointer alignment information

[Richard Earnshaw <rearnsha at arm dot com>]

> > Would it be reasonable to make the compiler not propagate alignment 
> > through a cast?  This would still leave the case where the alignment is 
> > derived from the layout of a struct.
> 
> I don't think that should be the default.  If this is really needed
> for some programs, you could make a new invokation options that 
> disables these alignment propagations, just like -fvolatile caters
> for programs that really mean volatile even they don't say so.

This makes sense, but taking that route, I think it then makes sense when 
the flag is unset to then turn off all alignment assumptions that are 
inferred via REGNO_POINTER_ALIGN.  The patch below does this.

Jeff, is the following patch OK to install?

<date>  Richard Earnshaw (rearnsha@arm.com)

	* toplev.c (flag_infer_align): New flag.
	(lang_independent_options): Add -finfer-align to list of flags.
	(main): Turn it on when optimizing.
	* flags.h (flag_infer_align): Declare it.
	* emit-rtl.c (mark_reg_pointer): Only set REGNO_POINTER_ALIGN if
	flag_infer_align.
	(init_emit): Note that alignments of stack and frame regs are well
	known, even if flag_infer_align is unset.
	* invoke.texi: Document -finfer-align.


Index: emit-rtl.c
===================================================================
RCS file: /egcs/carton/cvsfiles/egcs/gcc/emit-rtl.c,v
retrieving revision 1.45
diff -p -r1.45 emit-rtl.c
*** emit-rtl.c	1998/12/15 11:56:36	1.45
--- emit-rtl.c	1998/12/16 13:34:39
*************** mark_reg_pointer (reg, align)
*** 590,596 ****
  {
    REGNO_POINTER_FLAG (REGNO (reg)) = 1;
  
!   if (align)
      REGNO_POINTER_ALIGN (REGNO (reg)) = align;
  }
  
--- 590,596 ----
  {
    REGNO_POINTER_FLAG (REGNO (reg)) = 1;
  
!   if (align && flag_infer_align)
      REGNO_POINTER_ALIGN (REGNO (reg)) = align;
  }
  
*************** init_emit ()
*** 3370,3375 ****
--- 3370,3376 ----
    REGNO_POINTER_FLAG (VIRTUAL_CFA_REGNUM) = 1;
  
  #ifdef STACK_BOUNDARY
+   /* These alignments can safely be used even if flag_infer_align is off */
    REGNO_POINTER_ALIGN (STACK_POINTER_REGNUM) = STACK_BOUNDARY / BITS_PER_UNIT;
    REGNO_POINTER_ALIGN (FRAME_POINTER_REGNUM) = STACK_BOUNDARY / BITS_PER_UNIT;
    REGNO_POINTER_ALIGN (HARD_FRAME_POINTER_REGNUM)
Index: flags.h
===================================================================
RCS file: /egcs/carton/cvsfiles/egcs/gcc/flags.h,v
retrieving revision 1.22
diff -p -r1.22 flags.h
*** flags.h	1998/11/23 16:41:10	1.22
--- flags.h	1998/12/16 13:34:40
*************** extern int flag_regmove;
*** 456,461 ****
--- 456,467 ----
  
  /* Instrument functions with calls at entry and exit, for profiling.  */
  extern int flag_instrument_function_entry_exit;
+ 
+ /* Infer alignment of pointers from the context of their declaration,
+    for example a short inside a struct containing an int may well be
+    int-aligned.  */
+ extern int flag_infer_align;
+ 
  
  /* Other basic status info about current function.  */
  
Index: invoke.texi
===================================================================
RCS file: /egcs/carton/cvsfiles/egcs/gcc/invoke.texi,v
retrieving revision 1.80
diff -p -r1.80 invoke.texi
*** invoke.texi	1998/12/06 01:56:01	1.80
--- invoke.texi	1998/12/16 13:34:45
*************** in the following sections.
*** 159,164 ****
--- 159,165 ----
  -fschedule-insns2  -fstrength-reduce  -fthread-jumps
  -funroll-all-loops  -funroll-loops
  -fmove-all-movables  -freduce-all-givs -fstrict-aliasing
+ -finfer-align
  -O  -O0  -O1  -O2  -O3 -Os
  @end smallexample
  
*************** With @samp{-O}, the compiler tries to re
*** 2140,2150 ****
  time.
  
  When you specify @samp{-O}, the compiler turns on @samp{-fthread-jumps}
! and @samp{-fdefer-pop} on all machines.  The compiler turns on
! @samp{-fdelayed-branch} on machines that have delay slots, and
! @samp{-fomit-frame-pointer} on machines that can support debugging even
! without a frame pointer.  On some machines the compiler also turns
! on other flags.@refill
  
  @item -O2
  Optimize even more.  GNU CC performs nearly all supported optimizations
--- 2141,2152 ----
  time.
  
  When you specify @samp{-O}, the compiler turns on @samp{-fthread-jumps}
! @samp{-finfer-align} and @samp{-fdefer-pop} on all machines.  The
! compiler turns on @samp{-fdelayed-branch} on machines that have delay
! slots, and @samp{-fomit-frame-pointer} on machines that can support
! debugging even without a frame pointer.  On some machines the compiler
! also turns on other flags.
! @refill
  
  @item -O2
  Optimize even more.  GNU CC performs nearly all supported optimizations
*************** Always pop the arguments to each functio
*** 2208,2213 ****
--- 2210,2224 ----
  returns.  For machines which must pop arguments after a function call,
  the compiler normally lets arguments accumulate on the stack for several
  function calls and pops them all at once.
+ 
+ @item -finfer-align
+ Infer additional alignment information from the context of the
+ declaration of a pointer.  GCC can generate more efficient code for some
+ machines if a pointer is known to be more strictly aligned than the
+ minimum requirements for addressing a particular type.  However, some
+ non-portable programs may generate unexpected results if they use unsafe
+ pointer casts or assume too much about the layout of structures; using
+ @samp{-fno-infer-align} may allow these programs to work as expected.
  
  @item -fforce-mem
  Force memory operands to be copied into registers before doing
Index: toplev.c
===================================================================
RCS file: /egcs/carton/cvsfiles/egcs/gcc/toplev.c,v
retrieving revision 1.136
diff -p -r1.136 toplev.c
*** toplev.c	1998/12/13 00:59:38	1.136
--- toplev.c	1998/12/16 13:34:48
*************** int flag_strict_aliasing = 0;
*** 732,738 ****
--- 732,743 ----
  /* Instrument functions with calls at entry and exit, for profiling.  */
  int flag_instrument_function_entry_exit = 0;
  
+ /* Infer alignment of pointers from the context of their declaration,
+    for example a short inside a struct containing an int may well be
+    int-aligned.  */
+ int flag_infer_align = 0;
  
+ 
  /* Table of supported debugging formats.  */
  static struct
  {
*************** lang_independent_options f_options[] =
*** 935,940 ****
--- 940,947 ----
     "Suppress output of instruction numbers and line number notes in debugging dumps"},
    {"instrument-functions", &flag_instrument_function_entry_exit, 1,
     "Instrument function entry/exit with profiling calls"},
+   {"infer-align", &flag_infer_align, 1,
+    "Infer the alignment of pointers from the context of their declaration"},
    {"leading-underscore", &flag_leading_underscore, 1,
     "External symbols have a leading underscore" }
  };
*************** main (argc, argv)
*** 4586,4591 ****
--- 4593,4599 ----
      {
        flag_defer_pop = 1;
        flag_thread_jumps = 1;
+       flag_infer_align = 1;
  #ifdef DELAY_SLOTS
        flag_delayed_branch = 1;
  #endif