On 10/4/22 08:58, Peter Maydell wrote:
On Fri, 30 Sept 2022 at 23:10, Richard Henderson
<richard.hender...@linaro.org> wrote:
In preparation for TARGET_TB_PCREL, reduce reliance on absolute values.
Signed-off-by: Richard Henderson <richard.hender...@linaro.org>
---
target/arm/translate.c | 37 +++++++++++++++++++++----------------
1 file changed, 21 insertions(+), 16 deletions(-)
@@ -8368,7 +8372,8 @@ static bool trans_BLX_i(DisasContext *s, arg_BLX_i *a)
}
tcg_gen_movi_i32(cpu_R[14], s->base.pc_next | s->thumb);
store_cpu_field_constant(!s->thumb, thumb);
- gen_jmp(s, (read_pc(s) & ~3) + a->imm);
+ /* This difference computes a page offset so ok for TARGET_TB_PCREL. */
+ gen_jmp(s, (read_pc(s) & ~3) - s->pc_curr + a->imm);
Could we just calculate the offset of the jump target instead?
read_pc() returns s->pc_curr + a constant, so the s->pc_curr cancels
out anyway:
(read_pc(s) & ~3) - s->pc_curr + a->imm
==
(pc_curr + (s->thumb ? 4 : 8) & ~3) - pc_curr + imm
== pc_curr - pc_curr_low_bits - pc_curr + 4-or-8 + imm
== imm + 4-or-8 - low_bits_of_pc
That's then more obviously not dependent on the absolute value
of the PC.
Yes, this works:
- gen_jmp(s, (read_pc(s) & ~3) + a->imm);
+ /* This jump is computed from an aligned PC: subtract off the low bits. */
+ gen_jmp(s, jmp_diff(s, a->imm - (s->pc_curr & 3)));
r~
