Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference

[Sandra Loosemore]

On 02/19/2015 03:09 PM, Jakub Jelinek wrote:
>
> If you have hw where NULL is mapped and you know your code violates the
> C/C++ standards by placing objects at that address, simply do use
> the option that is designed for that purpose.

As I pointed out earlier, though, that won't help you if your program 
(perhaps implicitly) uses library code that's been built without the 
magic option.

I thought that declaring an object explicitly placed at address 0 as 
"volatile" ought to be sufficient to prevent writes to it from being 
optimized away and replaced with traps, but alas, that is not the case. 
  Also, a structure copy to such a volatile object is implicitly 
invoking the regular memcpy function without complaining about casting 
away volatile-ness, as an explicit call would do.  I've attached a 
couple quick test cases and .s output from arm-none-eabi-gcc -O2, using 
a mainline build from last night.

-Sandra
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	.cpu arm7tdmi
	.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, 1
	.eabi_attribute 30, 2
	.eabi_attribute 34, 0
	.eabi_attribute 18, 4
	.arm
	.syntax divided
	.file	"test.c"
	.text
	.align	2
	.global	f0
	.type	f0, %function
f0:
	@ Function supports interworking.
	@ Volatile: function does not return.
	@ args = 0, pretend = 0, frame = 0
	@ frame_needed = 0, uses_anonymous_args = 0
	@ link register save eliminated.
	mov	r3, #0
	str	r3, [r3]
	.inst	0xe7f000f0
	.size	f0, .-f0
	.align	2
	.global	fv
	.type	fv, %function
fv:
	@ Function supports interworking.
	@ Volatile: function does not return.
	@ args = 0, pretend = 0, frame = 0
	@ frame_needed = 0, uses_anonymous_args = 0
	@ link register save eliminated.
	mov	r3, #0
	str	r3, [r3]
	.inst	0xe7f000f0
	.size	fv, .-fv
	.align	2
	.global	fn
	.type	fn, %function
fn:
	@ Function supports interworking.
	@ args = 0, pretend = 0, frame = 0
	@ frame_needed = 0, uses_anonymous_args = 0
	@ link register save eliminated.
	mov	r3, #268435456
	ldr	r2, [r0]
	str	r2, [r3]
	bx	lr
	.size	fn, .-fn
	.ident	"GCC: (GNU) 5.0.0 20150219 (experimental)"
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	.cpu arm7tdmi
	.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, 1
	.eabi_attribute 30, 2
	.eabi_attribute 34, 0
	.eabi_attribute 18, 4
	.arm
	.syntax divided
	.file	"test2.c"
	.text
	.align	2
	.global	f0
	.type	f0, %function
f0:
	@ Function supports interworking.
	@ Volatile: function does not return.
	@ args = 0, pretend = 0, frame = 0
	@ frame_needed = 0, uses_anonymous_args = 0
	mov	r1, r0
	stmfd	sp!, {r4, lr}
	mov	r2, #108
	mov	r0, #0
	bl	memcpy
	.inst	0xe7f000f0
	.size	f0, .-f0
	.align	2
	.global	fv
	.type	fv, %function
fv:
	@ Function supports interworking.
	@ Volatile: function does not return.
	@ args = 0, pretend = 0, frame = 0
	@ frame_needed = 0, uses_anonymous_args = 0
	mov	r1, r0
	stmfd	sp!, {r4, lr}
	mov	r2, #108
	mov	r0, #0
	bl	memcpy
	.inst	0xe7f000f0
	.size	fv, .-fv
	.align	2
	.global	fn
	.type	fn, %function
fn:
	@ Function supports interworking.
	@ args = 0, pretend = 0, frame = 0
	@ frame_needed = 0, uses_anonymous_args = 0
	stmfd	sp!, {r4, lr}
	mov	r1, r0
	mov	r2, #108
	mov	r0, #268435456
	bl	memcpy
	ldmfd	sp!, {r4, lr}
	bx	lr
	.size	fn, .-fn
	.ident	"GCC: (GNU) 5.0.0 20150219 (experimental)"