https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114000
Jeffrey A. Law <law at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Blocks|120763 |
--- Comment #3 from Jeffrey A. Law <law at gcc dot gnu.org> ---
The combination of -fPIC and -mexplicit-relocs is crucial to tripping this bug.
That combination turns jumps from forms like this:
(jump_insn/j 34 28 29 7 (set (pc)
(label_ref:SI 13)) 443 {jump}
(nil)
-> 13)
Into a form like this:
(insn 34 28 35 7 (set (reg:SI 15 a5 [141])
(unspec:SI [
(label_ref:SI 13)
(const_int 1 [0x1])
(pc)
] UNSPEC_AUIPC)) 268 {auipcsi}
(expr_list:REG_EQUAL (unspec:SI [
(label_ref:SI 13)
(const_int 1 [0x1])
(pc)
] UNSPEC_AUIPC)
(insn_list:REG_LABEL_OPERAND 13 (nil))))
(insn 35 34 36 7 (set (reg:SI 15 a5 [141])
(lo_sum:SI (reg:SI 15 a5 [141])
(symbol_ref:SI (".LA1") [flags 0x2]))) 270 {*lowsi}
(expr_list:REG_EQUAL (label_ref:SI 13)
(nil)))
(jump_insn/j 36 35 29 7 (set (pc)
(reg:SI 15 a5 [141])) 444 {indirect_jumpsi}
(nil)
-> 13)
In the former form it's trivial to redirect the jump, for the latter is notably
more complex.
Jump redirection is critical for shrink-wrapping as we have a block (bb4) that
needs a prologue. We create a new block for the prologue (bb9) and we want to
redirect a jump to bb4 to instead jump to bb9.
Of course the jumps reaching bb4 have the complex form above. So simple
methods to change the jump fail. The last-ditch attempt uses an API that
assumes the jump we're changing is a fallthru path -- which is not the case
here we trigger an assertion.
Given this is dependent upon -mexplicit-relocs and nobody's using that in
practice as far as I know, I'm not going to spend further time on this problem
in the immediate future. I guess someone could add a diagnostic to the RISC-V
backend to indicate that explicit-relocs+PIC+reorder-blocks-and-partition isn't
supported.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120763
[Bug 120763] [meta-bug] Tracker for bugs to visit during weekly RISC-V meeting
