On 3/20/23 23:44, LIU Zhiwei wrote:
On 2023/3/21 14:06, Richard Henderson wrote:
On 3/20/23 21:53, LIU Zhiwei wrote:
TS_DEAD means we will release the register allocated for this temporary. But
at basic block ending, we can still use the allocted register.
Signed-off-by: LIU Zhiwei <zhiwei_...@linux.alibaba.com>
Test case?
I have run an Ubuntu image after this patch. It can boot.
That's surprising. I would have expected an assert with --enable-debug-tcg, but this
appears to be an oversight in tcg_reg_alloc_bb_end. We only validate the liveness data
for TEMP_EBB and TEMP_CONST, but not TEMP_TB or TEMP_GLOBAL.
But I can't find a direct test case. Because the IRs supported with flags TCG_OPF_BB_END
do not have input or output parameter, such as the set_label or br.
That's exactly why we want all GLOBAL and TB to be DEAD | MEM, so that they're saved back
to their home slots and released from their registers.
The register allocator for TCG does not work across extended basic blocks. Importantly,
if you have a forward branch like so:
g1 = func(a)
brcond ..., L1
stuff
g2 = func(b)
g1 = g2
discard g2
L1:
What value should g1->reg have at L1? The allocator does not do the global control flow
and allocation required to ensure that g1->reg is the same at the brcond and at the label.
Nominally, I would have expected one value for g1->reg at the branch, a different value
for g2->reg in the second BB, and for the assignment to steal g2->reg and move it to
g1->reg (per tcg_reg_alloc_mov of an IS_DEAD_ARG temp). Which would result in an
incorrect allocation at L1.
What are you attempting to do? Is this just guesswork?
r~