Hi Richard,
On 9/22/21 12:25, Richard Henderson wrote:
On 9/21/21 1:18 PM, WANG Xuerui wrote:
+ /* Test for PC-relative values that can be loaded faster. */
+ intptr_t pc_offset = val - (uintptr_t)s->code_ptr;
This isn't quite right for split r^x code buffer.
You should have seen this with --enable-debug-tcg...
You need pc_offset = tcg_pcrel_diff(s, (void *)val).
Indeed; I just realized TCG debugging isn't fully enabled with
--enable-debug only. Will fix in v3.
+ if (pc_offset == (int32_t)pc_offset) {
+ tcg_target_long lo = sextreg(pc_offset, 0, 12);
+ tcg_target_long hi = pc_offset - lo;
+ tcg_out_opc_pcaddu12i(s, rd, hi >> 12);
And... this doesn't quite work, right at the edges. If lo is
negative, hi can overflow out of range. There are a number of ways to
fix this. One is to extract the pieces and re-assemble to see if it
matches. Another is to rearrange the arithmetic just a little and use
PCALAU12I.
I actually wrote a small test program to test for this, but found no
overflow issues here; rather the tcg_out_opc_ori call below has
signedness problem (need to mask the low variable, which is signed, with
0xfff to avoid overwriting the opcode field). I think I'll add a
tcg_debug_assert here, but keep the logic intact.
+ tcg_target_long upper = (val >> 12) & 0xfffff;
+ tcg_target_long higher = (val >> 32) & 0xfffff;
Better to use extract64(val, 12, 20) and extract64(val, 32, 30).
Sure; but as the instructions perform sign-extension, thus taking signed
operands, sextract64 or the wrapped sextreg will do it.
r~
