[rv_plic] Work around sim/synth mismatch in Vivado #1364
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msfschaffner
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asb,
alistair23,
GregAC,
tjaychen,
eunchan,
imphil,
vogelpi,
sjgitty and
silvestrst
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Note that this patch requires #1365 to go in first, so do not merge yet (and if you build locally, make sure you pick both patches). |
imphil
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Jan 17, 2020
| assign is_tree[pa] = (sel) ? is_tree[c1] : is_tree[c0]; | ||
| assign id_tree[pa] = (sel) ? id_tree[c1] : id_tree[c0]; | ||
| assign max_tree[pa] = (sel) ? max_tree[c1] : max_tree[c0]; | ||
| assign is_tree[pa] = (sel & is_tree[c1]) | ((~sel) & is_tree[c0]); |
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Can you add a comment here that this is to work around a vivado bug, and reference the issue number? The code looks otherwise quite strange.
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I am still waiting for access to the Xilinx portal (I had to create a new account). In the meantime, let me add an explanation and a reference to our github issue.
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eunchan
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This works around a simulation synthesis mismatch that can be observed in Vivado 2018.3 and 2019.2 (other versions have not been tested). In particular, Vivado optimizes away the rv_plic_target module if the ternary statements are present within the two generate loops. The workaround consists of rewriting these MUX statements with bitwise ops and assign statements. The issue has been reported to Xilinx: https://forums.xilinx.com/t5/Synthesis/Simulation-Synthesis-Mismatch-with-Vivado-2018-3/m-p/1065923#M33849 Signed-off-by: Michael Schaffner <msf@opentitan.org>
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Ok I have added a comment with a link to the Xilinx issue. |
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Tracking issue: #1355 |
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This works around a simulation synthesis mismatch that can be observed in Vivado 2018.3 and 2019.2 (other versions have not been tested).
In particular, Vivado optimizes away the
rv_plic_targetmodule if the ternary statements are present within the two generate loops. The workaround consists of rewriting these MUX statements with bitwise ops and assign statements.The issue has been reported to Xilinx:
https://forums.xilinx.com/t5/Synthesis/Simulation-Synthesis-Mismatch-with-Vivado-2018-3/m-p/1065923#M33849
Signed-off-by: Michael Schaffner msf@opentitan.org