Invalid assembly generated at -O3

[fxcoudert]

Invalid assembly generated at -O3:

$ cat sound.c 
void toto (char audio[8192]) {
  int i;

  for (i = 0; i < 4095; i++)
    audio[i] = i / 8;
}
$ ./bin/gcc sound.c -c -O2
$ ./bin/gcc sound.c -c -O3
/var/folders/5b/7mrgq6j55b51p7ztj1r3gv400000gq/T//ccPBI4WH.s:36:14: error: immediate must be an integer in range [0, 255].
        movi    v0.8b, 0xfffffffffffffffe
                       ^

Reduced from gcc.c-torture/compile/sound.c
Diff of the generated assembly:

--- O2.s	2020-08-31 13:58:03.000000000 +0200
+++ O3.s	2020-08-31 13:58:12.000000000 +0200
@@ -6,16 +6,48 @@
 _toto:
 LFB0:
 	.cfi_startproc
-	mov	x1, 0
+	adrp	x1, lC0@GOTPAGE
+	add	x1, x1, lC0@PAGEOFF;momd
+	movi	v17.4s, 0x10
+	add	x2, x0, 4080
+	movi	v16.4s, 0x4
+	movi	v7.4s, 0x8
+	movi	v6.4s, 0xc
+	ldr	q2, [x1]
+	mov	x1, x0
 	.p2align 3,,7
 L2:
-	asr	w2, w1, 3
-	strb	w2, [x0, x1]
-	add	x1, x1, 1
-	cmp	x1, 4095
+	mov	v0.16b, v2.16b
+	add	v2.4s, v2.4s, v17.4s
+	add	v5.4s, v0.4s, v16.4s
+	add	v3.4s, v0.4s, v6.4s
+	add	v4.4s, v0.4s, v7.4s
+	xtn	v1.4h, v0.4s
+	xtn2	v1.8h, v5.4s
+	xtn	v0.4h, v4.4s
+	xtn2	v0.8h, v3.4s
+	ushr	v1.8h, v1.8h, 3
+	ushr	v0.8h, v0.8h, 3
+	xtn	v3.8b, v1.8h
+	xtn2	v3.16b, v0.8h
+	str	q3, [x1], 16
+	cmp	x2, x1
 	bne	L2
+	movi	v0.8b, 0xfffffffffffffffe
+	mov	w1, -1
+	str	w1, [x0, 4088]
+	strh	w1, [x0, 4092]
+	strb	w1, [x0, 4094]
+	str	d0, [x0, 4080]
 	ret
 	.cfi_endproc
 LFE0:
+	.const
+	.align	4
+lC0:
+	.word	0
+	.word	1
+	.word	2
+	.word	3
 	.ident	"GCC: (GNU) 11.0.0 20200829 (experimental)"
 	.subsections_via_symbols

[iains]

Invalid assembly generated at -O3:

$ cat sound.c 
void toto (char audio[8192]) {
  int i;

  for (i = 0; i < 4095; i++)
    audio[i] = i / 8;
}
$ ./bin/gcc sound.c -c -O2
$ ./bin/gcc sound.c -c -O3
/var/folders/5b/7mrgq6j55b51p7ztj1r3gv400000gq/T//ccPBI4WH.s:36:14: error: immediate must be an integer in range [0, 255].
        movi    v0.8b, 0xfffffffffffffffe
                       ^

I think there are a few instances of this in the test suite fails. I wonder if it's because Darwin's char is signed (where many platforms [most?] are unsigned) - so perhaps a masking operation is missing?, dunno, just speculating.

[fxcoudert]

Not sure I get what you mean, but changing the char to unsigned char (or signed char) in the source does not fix the issue.

[iains]

i / 8 is signed

if looks like the output has done something like / equivalent to...

printf ... %d ... (int) ((char) i / 8)

what happens if you do
audio[i] = (unsigned char) i / 8;

?

[fxcoudert]

If i is made unsigned char, or cast to that type as you showed, then the error goes away.

[iains]

should be fixed by
efda199