https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96015
--- Comment #9 from Sergei Trofimovich <slyfox at inbox dot ru> ---
(In reply to Martin Liška from comment #7)
> There's ASM diff in between GCC 9 and 10 version:
>
> diff -u good.s bad.s
> --- good.s 2020-07-01 15:04:58.315839436 +0200
> +++ bad.s 202
0-07-01 15:04:30.684040487 +0200
Hm, interesting! I think both these files are broken. Let's me try to elaborate
b annotating bad-bug.S. All the test does is to print result of comparison of
'2' and '1' stored in memory:
*lhs = 2; *rhs = 1;
int sign = *lhs - *rhs;
return sign;
But in bad-bug.S we never read from memory! (IPA is disabled to make functions
somewhat opaque):
"""
main:
ldi 2,%r28
stw %r2,-20(%r30)
ldo 64(%r30),%r30
stw %r28,-52(%r30) ; store '2' in RAM
ldo -56(%r30),%r19 ; get RAM address
ldi 1,%r28
ldi 0,%r24 ; arg3 = '0'
stw %r28,-56(%r30) ; store '1' in RAM
copy %r19,%r25 ; arg1 = &2
bl long_richcompare,%r2
ldo -52(%r30),%r26 ; arg0 = &1 (delay slot, executed before branch)
... (all ok so far)
long_richcompare:
stw %r2,-20(%r30)
stwm %r5,64(%r30)
copy %r26,%r5 ; arg0 = &1
stw %r4,-60(%r30)
copy %r25,%r4 ; arg1 = &2
stw %r3,-56(%r30)
bl yes,%r2
copy %r24,%r3 ; arg2 = 0 (delay slot, is it safe in general to
comiclr,= 0,%r28,%r0 ; if (!yes()) ...
b,n .L22 ; go to actual comparison
.L15:
ldi 0,%r28 ; fall through to 'return 0;' (not interesting)
.L3:
.L26:
ldw -84(%r30),%r2
ldw -60(%r30),%r4
ldw -56(%r30),%r3
bv %r0(%r2)
ldwm -64(%r30),%r5
.L22:
bl yes,%r2
nop
comib,=,n 0,%r28,.L26 ; if ( .. || !yes()) return 0; (not interesting)
ldi 0,%r28
comiclr,<< 5,%r3,%r0 ; check if 'arg3 < 5' to fit into jump table,
otherwise skip (nullify) next instruction and run .L3
b,n .L25 ; handle jump table
.L12:
b .L3 ; return 0; (not interesting, fall through)
ldi 1,%r28
.L25:
ldil L'.L8,%r28 ;
ldo R'.L8(%r28),%r28 ; load jump table address
ldwx,s %r3(%r28),%r28 ; load target at .L8[arg2 * 4]
bv,n %r0(%r28) ; jump on target, should be .L12
.section .rodata
.align 4
.L8:
.begin_brtab
.word .L12
.word .L15
.word .L12
.word .L15
.word .L12
.word .L12
.end_brtab
"""
Note: during the whole execution at no point in time 'long_richcompare()' tried
to dereference arg0 and arg1 inputs (%r4, %r5 registers).
For comparison compiling with -O1 keeps the loads around:
good-bug.S:
"""
main: ; same as above
stw %r2,-20(%r30)
ldo 64(%r30),%r30
ldi 2,%r28
stw %r28,-56(%r30)
ldi 1,%r28
ldo -52(%r30),%r19
stw %r28,-52(%r30)
ldi 0,%r24
copy %r19,%r25
bl long_richcompare,%r2
ldo -56(%r30),%r26
...
long_richcompare:
stw %r2,-20(%r30)
stwm %r5,64(%r30)
stw %r4,-60(%r30)
stw %r3,-56(%r30)
copy %r26,%r4 ; arg0
copy %r25,%r3 ; arg1
bl yes,%r2
copy %r24,%r5 ; arg2
or,= %r28,%r0,%r28 ; result = 0
b,n .L11 ;
.L2:
ldw -84(%r30),%r2
.L12:
ldw -60(%r30),%r4
ldw -56(%r30),%r3
bv %r0(%r2) ; return
ldwm -64(%r30),%r5
.L11:
bl yes,%r2
nop
movb,= %r28,%r28,.L12 ; if(!yes()) return ...
ldw -84(%r30),%r2
comb,=,n %r3,%r4,.L9 ; if(arg0 == arg1) (at branch) diff = 0;
ldw 0(%r4),%r28
ldw 0(%r3),%r19
sub %r28,%r19,%r28 ; diff = *arg0 - *arg1
comiclr,<> 0,%r28,%r0
ldi 1,%r28
.L4:
comiclr,>>= 5,%r5,%r0
b,n .L6
ldil L'.L7,%r19
ldo R'.L7(%r19),%r19
ldwx,s %r5(%r19),%r19
bv,n %r0(%r19) ; handle jump table, at .L8
.section .rodata
.align 4
.L7:
.begin_brtab
.word .L8
.word .L10
.word .L6
.word .L10
.word .L6
.word .L6
.end_brtab
.text
.L9:
b .L4
ldi 0,%r28
.L8:
comiclr,<> 0,%r28,%r28 ; if (result == 0)
ldi 1,%r28 ; result = 1;
b .L12 ; return
ldw -84(%r30),%r2
.L6:
comiclr,<> 0,%r28,%r28
ldi 1,%r28
b .L12
ldw -84(%r30),%r2
.L10:
b .L2
ldi 0,%r28
"""
