Sender: chris AT mindspring DOT com
Message-ID: <38921CD6.2A725779@ix.netcom.com>
Date: Fri, 28 Jan 2000 14:48:54 -0800
From: Chris Sears <cbsears AT ix DOT netcom DOT com>
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To: pgcc AT delorie DOT com
Subject: pgcc and egcs alignment -- function, basic block and string
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I am using pgcc version 2.95.2 and egcs version 2.95.1

    Reading specs from
/usr/local/pgcc/lib/gcc-lib/i686-pc-linux-gnu/pgcc-2.95.2/specs
    gcc version pgcc-2.95.2 19991024 (release)

    Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.95.1/specs
    gcc version 2.95.1 19990816/Linux (release)

This is my pgcc compile line:

        pgcc -D__KERNEL__ -I/home/chris/linux/include
            -D__SMP__ -Wall -Wstrict-prototypes -O2
            -fomit-frame-pointer -fno-strict-aliasing
            -pipe -fno-strength-reduce
            -mpreferred-stack-boundary=2 -DCPU=686
            -march=i686   -S arch/i386/kernel/ioport.c
            -o arch/i386/kernel/ioport.s

In pgcc functions are being aligned to 16 byte boundaries.
Shouldn't this be alignment to a cacheline, .align 32?

        .text
         .align 16
         .type  set_bitmap,@function
        set_bitmap:
         pushl %ebp
         pushl %edi

In pgcc some basic blocks (loops?) are being aligned.
These 16 byte blocks are ifetch blocks.
Quoting Agner Fog, "While aligning data is always important,
aligning code is not necessary on the PPlain and PMMX."
He means with respect to instruction fetch, not cache line.
Is this alignment a good idea?  It seems unnecessary from
a processor point of view and it seems to increase
the cache footprint.  The p2align 4,,7 means align min(2^4,7)
and it means that there may be some padded nop instructions.
This is a COST for ifetch alignment in addition to the
cache footprint.

         cmpl $31,%ebx
         jle .L1476
         .p2align 4,,7
        .L1471:
         movl %edx,(%ebp)
         addl $4,%ebp

In pgcc strings are being aligned to cache lines.
But is alignment even necessary for strings?
Data structures yes, strings no.

         .file "ioport.c"
         .version "01.01"
        gcc2_compiled.:
        .section .rodata
        .LC0:
         .string "eip: %p\n"
         .align 32
        .LC1:
         .string "/home/chris/linux/include/asm/spinlock.h"

egcs has the same string (32) and basic block alignment (.p2align 4,,7)
But it uses .align 4 (!) for functions.  I might point out that the gas
documentation has a bug in the .align description saying that the
operand is like the .p2align operand, the number of bits to shift.

So in summary, I think that functions should be aligned to cache lines
and that basic blocks and strings should not be aligned at all.

Chris Sears
cbsears AT ix DOT netcom DOT com