Created attachment 43915 [details]
example .c file - check for invalid index optimized out of foo() only

The following code, when compiled with gcc-6.4 or gcc-7.2 and -fsanitize=object-size and -O2 or higher creates unnecessary code to ensure a valid index.  I'm configured for aarch64, but also see this with armv7.

When indexing a 256-element array with either a uint8_t variable or an integer variable shifted right by 24 or more, gcc inserts code to ensure a valid index.  That code DOES get optimized out when using an integer variable with a smaller or variable shift count and an explicit mask with 0xff.

With the index clearly being an unsigned 8-bit value, the check for > 256 should not be needed.  Even explicitly and'ing with 0xff does not cause the check to get optimized out when using an 8-bit datatype.

lut[(*pi >> 23) & 0xff]);  // good
lut[(*pi >> 24) & 0xff]);  // extra check
lut[(*pi >> 25) & 0xff]);  // extra check
lut[(*pi >> 24) & 0x1]);   // extra check
lut[*pb]		   // extra check
lut[*pb & 0xff];           // extra check
lut[*pb >> 7];             // extra check
lut[*pb >> 8];             // good - loads first element

/bonus/scratch/gcc64/poky/proj/tmp/work/aarch64-poky-linux/graph/*/recipe-sysroot-native/usr/bin/aarch64-poky-linux/aarch64-poky-linux-gcc  -mcpu=cortex-a53   --sysroot=/bonus/scratch/gcc64/poky/proj/tmp/work/aarch64-poky-linux/graph/*/recipe-sysroot -S -O4  -fsanitize=object-size  -fno-sanitize-recover -fsanitize-undefined-trap-on-error -Wall -Wextra   ~/sanitize.c

gcc-7.2 assembly output follows:
	.arch armv8-a+crc
	.file	"sanitize.c"
	.text
	.align	2
	.p2align 4,,15
	.global	foo
	.type	foo, %function
foo:
	ldrb	w1, [x0, 1]
	adrp	x0, lut
	add	x0, x0, :lo12:lut
	ldr	w0, [x0, x1, lsl 2]
	ret
	.size	foo, .-foo
	.align	2
	.p2align 4,,15
	.global	bar
	.type	bar, %function
bar:
	ldrb	w2, [x0, 3]
	adrp	x1, lut
	add	x0, x1, :lo12:lut
	add	x3, x0, x2, lsl 2
	sub	x0, x3, x0
	add	x0, x0, 4
	cmp	x0, 1024
	bhi	.L6
.L4:
	add	x1, x1, :lo12:lut
	ldr	w0, [x1, x2, lsl 2]
	ret
.L6:
	add	x0, x3, x0
	cmp	x3, x0
	bhi	.L4
	brk #1000
	.size	bar, .-bar
	.align	2
	.p2align 4,,15
	.global	baz
	.type	baz, %function
baz:
	ldrb	w2, [x0]
	adrp	x1, lut
	add	x0, x1, :lo12:lut
	add	x3, x0, x2, uxtb 2
	sub	x0, x3, x0
	add	x0, x0, 4
	cmp	x0, 1024
	bhi	.L10
.L8:
	add	x1, x1, :lo12:lut
	ldr	w0, [x1, x2, lsl 2]
	ret
.L10:
	add	x0, x3, x0
	cmp	x3, x0
	bhi	.L8
	brk #1000
	.size	baz, .-baz
	.ident	"GCC: (GNU) 7.2.0"
	.section	.note.GNU-stack,"",@progbits