get_native_u2 & friends

[Gary Benson]

Hi Ed,

Sorry I haven't looked into this yet.  I filed a bug for it
(http://icedtea.classpath.org/bugzilla/show_bug.cgi?id=301)
to make sure it doesn't get lost.

Cheers,
Gary

Edward Nevill wrote:
> Hi all,
> 
> I think the best thing to do with get_native_u2 and friends is to
> let the compiler decide how to access unaligned data. Most modern
> compilers have some facility for doing this.  In gcc you can use
> __attribute__((packed)) as follows.
> 
> typedef union unaligned {
> 	unsigned	u;
> 	unsigned short	us;
> 	unsigned long long ul;
> } __attribute__((packed)) unaligned;
> 
> unsigned short get_native_u2(unaligned *p)
> {
> 	return p->us;
> }
> 
> unsigned get_native_u4(unaligned *p)
> {
> 	return p->u;
> }
> 
> unsigned long long get_native_u8(unaligned *p)
> {
> 	return p->ul;
> }
> 
> Below is the code generated for ARM and X86. Note that in the X86
> case it just does the access since X86 allows unaligned accesses
> whereas for ARM it goes ahead and doesbyte loads.
> 

> If on some architechture it is better to test the alignment and then
> do word/halfword loads if the pointer is aligned and byte loads
> otherwise then hopefully the compiler will know the best code to
> generate rarther than us trying to second guess what is best on
> individual architectures.
> 
> Also, in many case these functions are called when it is known that
> the data is aligned as in this example from _tableswitch...
> 
>       CASE(_tableswitch): {
>           jint* lpc  = (jint*)VMalignWordUp(pc+1);
>           int32_t  key  = STACK_INT(-1);
>           int32_t  low  = Bytes::get_Java_u4((address)&lpc[1]);
>           int32_t  high = Bytes::get_Java_u4((address)&lpc[2]);
> 
> Maybe it is worth having get_Java_u4() and get_Java_u4_unaligned()?
> 
> Regards,
> Ed.
> 
> 
> --- x86.s --------------------------------------------------------
> 	.file	"test.c"
> 	.text
> 	.p2align 4,,15
> .globl get_native_u2
> 	.type	get_native_u2, @function
> get_native_u2:
> 	pushl	%ebp
> 	movl	%esp, %ebp
> 	movl	8(%ebp), %eax
> 	popl	%ebp
> 	movzwl	(%eax), %eax
> 	ret
> 	.size	get_native_u2, .-get_native_u2
> 	.p2align 4,,15
> .globl get_native_u4
> 	.type	get_native_u4, @function
> get_native_u4:
> 	pushl	%ebp
> 	movl	%esp, %ebp
> 	movl	8(%ebp), %eax
> 	popl	%ebp
> 	movl	(%eax), %eax
> 	ret
> 	.size	get_native_u4, .-get_native_u4
> 	.p2align 4,,15
> .globl get_native_u8
> 	.type	get_native_u8, @function
> get_native_u8:
> 	pushl	%ebp
> 	movl	%esp, %ebp
> 	movl	8(%ebp), %eax
> 	popl	%ebp
> 	movl	4(%eax), %edx
> 	movl	(%eax), %eax
> 	ret
> 	.size	get_native_u8, .-get_native_u8
> 	.ident	"GCC: (GNU) 4.2.4 (Ubuntu 4.2.4-1ubuntu3)"
> 	.section	.note.GNU-stack,"", at progbits
> --- arm.s -------------------------------------------------------------
> 	.arch armv5t
> 	.fpu softvfp
> 	.eabi_attribute 20, 1
> 	.eabi_attribute 21, 1
> 	.eabi_attribute 23, 3
> 	.eabi_attribute 24, 1
> 	.eabi_attribute 25, 1
> 	.eabi_attribute 26, 2
> 	.eabi_attribute 30, 2
> 	.eabi_attribute 18, 4
> 	.file	"test.c"
> 	.text
> 	.align	2
> 	.global	get_native_u2
> 	.type	get_native_u2, %function
> get_native_u2:
> 	@ args = 0, pretend = 0, frame = 0
> 	@ frame_needed = 0, uses_anonymous_args = 0
> 	@ link register save eliminated.
> 	ldrb	r3, [r0, #1]	@ zero_extendqisi2
> 	ldrb	r0, [r0, #0]	@ zero_extendqisi2
> 	orr	r0, r0, r3, asl #8
> 	bx	lr
> 	.size	get_native_u2, .-get_native_u2
> 	.align	2
> 	.global	get_native_u4
> 	.type	get_native_u4, %function
> get_native_u4:
> 	@ args = 0, pretend = 0, frame = 0
> 	@ frame_needed = 0, uses_anonymous_args = 0
> 	@ link register save eliminated.
> 	ldrb	r1, [r0, #1]	@ zero_extendqisi2
> 	ldrb	r3, [r0, #0]	@ zero_extendqisi2
> 	ldrb	r2, [r0, #2]	@ zero_extendqisi2
> 	ldrb	r0, [r0, #3]	@ zero_extendqisi2
> 	orr	r3, r3, r1, asl #8
> 	orr	r3, r3, r2, asl #16
> 	orr	r0, r3, r0, asl #24
> 	bx	lr
> 	.size	get_native_u4, .-get_native_u4
> 	.align	2
> 	.global	get_native_u8
> 	.type	get_native_u8, %function
> get_native_u8:
> 	@ args = 0, pretend = 0, frame = 0
> 	@ frame_needed = 0, uses_anonymous_args = 0
> 	@ link register save eliminated.
> 	stmfd	sp!, {r4, r5, r6}
> 	ldrb	r5, [r0, #1]	@ zero_extendqisi2
> 	ldrb	r6, [r0, #5]	@ zero_extendqisi2
> 	ldrb	r3, [r0, #0]	@ zero_extendqisi2
> 	ldrb	ip, [r0, #2]	@ zero_extendqisi2
> 	ldrb	r1, [r0, #4]	@ zero_extendqisi2
> 	ldrb	r2, [r0, #6]	@ zero_extendqisi2
> 	ldrb	r4, [r0, #7]	@ zero_extendqisi2
> 	ldrb	r0, [r0, #3]	@ zero_extendqisi2
> 	orr	r3, r3, r5, asl #8
> 	orr	r1, r1, r6, asl #8
> 	orr	r3, r3, ip, asl #16
> 	orr	r1, r1, r2, asl #16
> 	orr	r0, r3, r0, asl #24
> 	orr	r1, r1, r4, asl #24
> 	ldmfd	sp!, {r4, r5, r6}
> 	bx	lr
> 	.size	get_native_u8, .-get_native_u8
> 	.ident	"GCC: (Ubuntu 4.3.3-1ubuntu1) 4.3.3"
> 	.section	.note.GNU-stack,"",%progbits
> 
> 

-- 
http://gbenson.net/