RFR: 8322816: RISC-V: Incorrect guarantee in patch_vtype

[Kim Barrett]

On Tue, 2 Jan 2024 08:56:08 GMT, Fei Yang <fyang at openjdk.org> wrote:

>> Please review this change that fixes a test for a guarantee.  This also
>> removes a -Wparentheses warning when those are enabled (which is how the
>> problem was discovered).
>> 
>> The problem is that operator precedence groups the sub-expressions differently
>> than intended. The fix is to override the operator precedence by adding
>> parentheses to achieve the intended grouping.
>> 
>> Testing: Local (linux-x64) cross-build for linux-riscv with this change plus
>> -Wparentheses enabled and other changes to allow that to work.
>> 
>> Requesting someone from the riscv porters to properly test this.
>
> src/hotspot/cpu/riscv/assembler_riscv.hpp line 1160:
> 
>> 1158: #define patch_vtype(hsb, lsb, vlmul, vsew, vta, vma, vill)   \
>> 1159:     if (vill == 1) {                                         \
>> 1160:       guarantee((vlmul | vsew | vta | vma) == 0,             \
> 
> I see the `vill` parameter is always false in current code, which means this guarantee never gets excecuted. And I don't think we would make use of the `vill` field of vtype in future. So I personally perfer to remove this guarantee and its enclosing if block for now.

Rather than removing the guarantee, wouldn't it be better to guarantee/assert `vill == 0`?
Although looking at uses, that argument is a bool, so it should be `guarantee(!vill, ...)`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17215#discussion_r1439970844