Hi Richard,
As part of implementing the new O32 FPXX ABI I am making use of the
HARD_REGNO_CALL_PART_CLOBBERED macro to allow odd-numbered floating-point
registers to be considered as 'normally' callee-saved but call clobbered if
they
are being used to hold SImode or SFmode data. The macro is implemented as:
/* Odd numbered single precision registers are not considered call saved
for O32 FPXX as they will be clobbered when run on an FR=1 FPU. */
#define HARD_REGNO_CALL_PART_CLOBBERED(REGNO, MODE) \
(TARGET_FLOATXX && ((MODE) == SFmode || (MODE) == SImode) \
&& FP_REG_P (REGNO) && (REGNO & 1))
IRA and LRA appear to work correctly w.r.t. HARD_REGNO_CALL_PART_CLOBBERED and
I
get the desired O32 FPXX ABI behaviour. However when writing a number of tests
for this I triggered some optimisations (in particular regcprop) which ignored
the fact that the odd-numbered single-precision registers are clobbered across
calls and essentially undid the work IRA/LRA did in treating the register as
clobbered. The reason for regcprop ignoring the call-clobbered nature of these
registers is that it simply does not check. The test for call-clobbered
registers solely relies on regs_invalidated_by_call which is (and cannot be)
aware of the HARD_REGNO_CALL_PART_CLOBBERED macro as it has no information about
what mode registers are in when it is used. A proposed fix is inline below for
this specific issue.
diff --git a/gcc/regcprop.c b/gcc/regcprop.c
index 101de76..cb2937c 100644
--- a/gcc/regcprop.c
+++ b/gcc/regcprop.c
@@ -1030,8 +1030,10 @@ copyprop_hardreg_forward_1 (basic_block bb, struct
value_data *vd)
}
}
- EXECUTE_IF_SET_IN_HARD_REG_SET (regs_invalidated_by_call, 0, regno,
hrsi)
- if (regno < set_regno || regno >= set_regno + set_nregs)
+ for (regno = 0; regno < FIRST_PSEUDO_REGISTER; regno++)
+ if ((TEST_HARD_REG_BIT (regs_invalidated_by_call, regno)
+ || HARD_REGNO_CALL_PART_CLOBBERED (regno, vd->e[regno].mode))
+ && (regno < set_regno || regno >= set_regno + set_nregs))
kill_value_regno (regno, 1, vd);
/* If SET was seen in CALL_INSN_FUNCTION_USAGE, and SET_SRC
The problem is that there are several other passes that solely rely on
regs_invalidated_by_call to determine call-clobbered status and will therefore
make the mistake. Some of these passes simply don't have mode information
around
when handling call-clobbered registers which leaves me a little unsure of the
best solution in those cases. I would expect that being over-cautious and
always
marking a potentially clobbered register as clobbered seems like one option but
there is a risk that doing so could lead to legitimate use of a callee-saved
register (in a mode that is not part clobbered) to be broken. Essentially I
would propose introducing another register set 'regs_maybe_invalidated_by_call'
that includes all reg_invalidated_by_call and anything
HARD_REGNO_CALL_PART_CLOBBERED reports true for when checking all registers
against all modes. Wherever call-clobbered information is required but mode
information is unavailable then regs_maybe_invalidated_by_call would then be
used. As I said though there are probably some corner cases to handle too.
I don't quite have the O32 FPXX patches ready to send out yet but this issue is
relevant to all architectures using HARD_REGNO_CALL_PART_CLOBBERED, presumably
nobody has hit it yet though.
Regards,
Matthew
