Delay flush by one cycle #762
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piotro888
reviewed
Nov 26, 2024
Comment on lines
-183
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+182
| # Condition is used to avoid FRAT locking during normal operation | ||
| with condition(m) as cond: | ||
| with cond(commit): | ||
| retire_instr(rob_entry) | ||
| with cond(): | ||
| # Not using default condition, because we want to block if branch is not ready | ||
| flush_instr(rob_entry) | ||
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| m.d.comb += core_flushing.eq(1) | ||
| with m.If(commit): | ||
| retire_instr(rob_entry) |
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This is not valid - when instruction is an exception with commit=0 (like synchronous exceptions),
RF is not freed (and not pushed to Free RF), but instruction is retired from ROB, and TRAP_FLUSH would flush next instruction.
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Indeed, you're right - this was not thought out enough. Still, this means we have a blind spot in tests for this.
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This PR simplifies retirement by delaying the start of flushing. This eliminates one use of
conditionand makes theflushingsignal registered. Looking at benchmarks:is not significantly impactedsomehow didn't change at all.