2013/11/21 Uros Bizjak <ubiz...@gmail.com>: > On Wed, Nov 20, 2013 at 5:33 PM, Ilya Enkovich <enkovich....@gmail.com> wrote: >>>> CM_MEDIUM has unlimited data size. >>>> >>>> i386-opts.h: CM_MEDIUM, /* Assumes code fits in the low 31 >>>> bits; data unlimited. */ >>>> >>>> The x86_64_zext_immediate_operand allows _address_ to be loaded by >>>> movl. The @SIZE reloc returns object size, which is unlimited and has >>>> no connection to its address. For CM_MEDIUM, >>>> x86_64_zext_immediate_operand allows: >>>> >>>> return (ix86_cmodel == CM_SMALL >>>> || (ix86_cmodel == CM_MEDIUM >>>> && !SYMBOL_REF_FAR_ADDR_P (op))); >>> >>> Yes, but large symbols never have SYMBOL_FLAG_FAR_ADDR set. >> >> Sorry, I mean all large symbols have this flag set. > > This flag marks the fact that object size is bigger than > -mlarge-data-threshold and goes into large section. It is true that > UInt option argument currently overflows at 4G, and consequently all > sizes larger than 4G go to large sections, but IMO, we should not rely > on this. The specification says unlimited data size, so we have to be > prepared for this. > > The reason I am against checking "Z" constraint is that "@SIZE" > relocation returns _size_ of the object (which is only remotely > connected to its address), while "Z" checks the address of the object. > So, it looks to me that this check would be conceptually wrong. > > The right place to "fix" movabs is in a linker relaxation pass that > can check the real size and change movabs to movl if the size fits > 32bits. > > As a side note, perhaps generic option subsystem should reject > overflows as e.g. -mlarge-data-threshold=5368709120 when 32bit UInt is > used for option argument. The effects of overflow are surprising.
Hi Uros, Thanks for explanations! I'll make a fix for the patch. BTW Pointer Bounds Checker feature does not go to 4.9 and therefore this size relocation instructions is not required in 4.9. Suppose it should go into 4.10 because doubt this feature is useful for anyone except me. This also affects all MPX instructions. With no checker in 4.9, no one actually uses them. Please tell me if I should pull them back from trunk. Thanks, Ilya > > Uros.
