-fomit-frame-pointer interaction with alloca - egcs-1.0.1

[John Hughes]

The "-fomit-frame-pointer" option interacts poorly with alloca.

If a function contains a call to alloca then -fomit-frame-pointer seems to
get turned off.

This is a pity.

What's worse is that some parts of egcs don't seem to know and carry on as
if -fomit-frame pointer was still in effect, generating unnecessary stack
pointer manipulations.

For example, the following test program:

	int test (int p) {
	        printf ("who\n");
	        {
	                char *s = alloca (p);
	                printf ("boo\n");
	        }
	        printf ("foo\n");
	        return 1;
	}

Generates code like this:

        .file   "test.c"                        .file   "test.c"
        .version        "01.01"                 .version        "01.01"
/ GNU C version egcs-2.90.23 980102 (   / GNU C version egcs-2.90.23 980102 (
/ options passed:  -O -fomit-frame-po | / options passed:  -O
/ options enabled:  -fdefer-pop -fomi | / options enabled:  -fdefer-pop -fthr
/ -fpeephole -ffunction-cse -finline  | / -finline -fkeep-static-consts -fpcc
/ -fpcc-struct-return -fcommon -fverb | / -fgnu-linker -falias-check -fargume
/ -fargument-alias -m80387 -mhard-flo | / -mno-soft-float -mieee-fp -mfp-ret-
/ -mfp-ret-in-387 -mschedule-prologue | / -march=pentium

gcc2_compiled.:                         gcc2_compiled.:
.section        .rodata                 .section        .rodata
.LC0:                                   .LC0:
        .string "who\n"                         .string "who\n"
.LC1:                                   .LC1:
        .string "boo\n"                         .string "boo\n"
.LC2:                                   .LC2:
        .string "foo\n"                         .string "foo\n"
.text                                   .text
        .align 4                                .align 4
.globl test                             .globl test
        .type    test,@function                 .type    test,@function
test:                                   test:
        pushl %ebp                              pushl %ebp
        movl %esp,%ebp                          movl %esp,%ebp
        pushl %ebx                              pushl %ebx
        movl 8(%ebp),%ebx                       movl 8(%ebp),%ebx
        pushl $.LC0                             pushl $.LC0
        call printf                             call printf
        addl $3,%ebx                            addl $3,%ebx
        andl $-4,%ebx                           andl $-4,%ebx
        addl $4,%esp                            addl $4,%esp
        subl %ebx,%esp                          subl %ebx,%esp
        pushl $.LC1                             pushl $.LC1
        call printf                             call printf
        pushl $.LC2                             pushl $.LC2
        call printf                             call printf
        movl $1,%eax                            movl $1,%eax
        addl $8,%esp                  <
        movl -4(%ebp),%ebx                      movl -4(%ebp),%ebx
        leave                                   leave
        ret                                     ret
.Lfe1:                                  .Lfe1:
        .size    test,.Lfe1-test                .size    test,.Lfe1-test
        .ident  "GCC: (GNU) egcs-2.90           .ident  "GCC: (GNU) egcs-2.90
	
Note the unnecessary "addl $8,%esp" in the code compiled with 
"-fomit-frame-pointer".

(gcc 2.7.2 shows the same problem).

-- 
John Hughes <john@AtlanTech.COM>,
	Atlantic Technologies Inc.	Tel: +33-1-43204546
	24 rue Montbrun,		 Fax: +33-1-43204579
	75014 PARIS.