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From: | Richard Henderson |
Subject: | Re: [PATCH] tcg/tcg: Avoid TS_DEAD for basic block ending |
Date: | Tue, 21 Mar 2023 08:39:23 -0700 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.8.0 |
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_liu@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~
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