https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70885
Bug ID: 70885
Summary: [SH] Use MSB pointer-tagging for pointer-to-member
representation
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: olegendo at gcc dot gnu.org
Target Milestone: ---
Currently there are two available pointer-to-member representation options:
ptrmemfunc_vbit_in_pfn,
ptrmemfunc_vbit_in_delta
where ptrmemfunc_vbit_in_pfn stores the "is a virtual function" pointer tag in
the LSB of the pointer (assuming 16-bit aligned functions).
When using the tagged pointer to make the function call, the tag needs to be
tested and the pointer converted into a valid address. On SH this currently
looks like this:
struct test_class
{
int x;
void func (void);
};
void test (test_class* c, void (test_class::*f)(void) )
{
(c->*f) ();
}
compiled with -O2:
mov r5,r0 // r5 = r0 = __pfn field
tst #1,r0 // test LSB
add r6,r4
bt .L17 // if LSB not set, goto L17
mov.l @r4,r5 // when here, LSB is set
add r5,r0
add #-1,r0 // subtract 1 to clear LSB
mov.l @r0,r0
.L17:
jmp @r0
nop
At least on SH, it's better to do store pointer tags in the MSB. So that the
tagged pointer value becomes:
tagged_ptr_val = (ptr_val >> 1) | (tag_bit << 31)
This allows for more efficient tag test and pointer value restoration, the code
could look like:
mov r5,r0
shll r5 // __pfn << 1, MSB -> T bit
add r6,r4
bf .L17 // if MSB not set, goto L17
mov.l @r4,r5
add r5,r0
mov.l @r0,r0
.L17:
jmp @r0
nop
... which saves one instruction.
To enable that, there should be another option "ptrmemfunc_vbit_in_pfn_msb"
along with some target specific m-option to enable this as well as a configure
option to control the default setting.
