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Add missing tail cost.

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This commit is contained in:
Thomas Harte 2022-06-02 12:26:25 -04:00
parent 6dd89eb0d7
commit f8e933438e
1 changed files with 8 additions and 6 deletions
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14
Processors/68000Mk2/Implementation/68000Mk2Implementation.hpp
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@ -2547,7 +2547,7 @@ template <bool did_overflow> void ProcessorBase::did_divu(uint32_t dividend, uin
} }
if(did_overflow) { if(did_overflow) {
dynamic_instruction_length_ = 3; // Just a quick nn n, and then on to prefetch. dynamic_instruction_length_ = 3; // Covers the nn n to get into the loop.
return; return;
} }
@ -2556,13 +2556,15 @@ template <bool did_overflow> void ProcessorBase::did_divu(uint32_t dividend, uin
// since this is a classic divide algorithm, but would rather that // since this is a classic divide algorithm, but would rather that
// errors produce incorrect timing only, not incorrect timing plus // errors produce incorrect timing only, not incorrect timing plus
// incorrect results. // incorrect results.
dynamic_instruction_length_ = 3; // Covers the nn n to get into the loop. dynamic_instruction_length_ =
3 + // nn n to get into the loop;
30 + // nn per iteration of the loop below;
3; // n nn upon completion of the loop.
divisor <<= 16; divisor <<= 16;
for(int c = 0; c < 15; ++c) { for(int c = 0; c < 15; ++c) {
if(dividend & 0x80000000) { if(dividend & 0x8000'0000) {
dividend = (dividend << 1) - divisor; dividend = (dividend << 1) - divisor;
dynamic_instruction_length_ += 2; // The fixed nn iteration cost.
} else { } else {
dividend <<= 1; dividend <<= 1;
@ -2570,9 +2572,9 @@ template <bool did_overflow> void ProcessorBase::did_divu(uint32_t dividend, uin
// and test the sign of the result, but this is easier to follow: // and test the sign of the result, but this is easier to follow:
if (dividend >= divisor) { if (dividend >= divisor) {
dividend -= divisor; dividend -= divisor;
dynamic_instruction_length_ += 3; // i.e. the original nn plus one further n before going down the MSB=0 route. dynamic_instruction_length_ += 1; // i.e. the original nn plus one further n before going down the MSB=0 route.
} else { } else {
dynamic_instruction_length_ += 4; // The costliest path (since in real life it's a subtraction and then a step dynamic_instruction_length_ += 2; // The costliest path (since in real life it's a subtraction and then a step
// back from there) — all costs accrue. So the fixed nn loop plus another n, // back from there) — all costs accrue. So the fixed nn loop plus another n,
// plus another one. // plus another one.
} }