changeset abdcb0389716 in /z/repo/m5
details: http://repo.m5sim.org/m5?cmd=changeset;node=abdcb0389716
description:
X86: Finally fix a division corner case.
When doing an unsigned 64 bit division with a divisor that has its most
significant bit set, the division code would spill a bit off of the end
of a
uint64_t trying to shift the dividend into position. This change adds
code
that handles that case specially by purposefully letting it spill and
then
going ahead assuming there was a 65th one bit.
diffstat:
src/arch/x86/isa/microops/regop.isa | 37 +++++++++++++++++++++++++++++--------
1 files changed, 29 insertions(+), 8 deletions(-)
diffs (48 lines):
diff -r edde97a6ea7c -r abdcb0389716 src/arch/x86/isa/microops/regop.isa
--- a/src/arch/x86/isa/microops/regop.isa Sun Apr 18 21:33:59 2010 -0700
+++ b/src/arch/x86/isa/microops/regop.isa Sun May 02 00:39:29 2010 -0700
@@ -612,15 +612,36 @@
//If we overshot, do nothing. This lets us unrool division loops a
//little.
if (remaining) {
- //Shift in bits from the low order portion of the dividend
- while(dividend < divisor && remaining) {
- dividend = (dividend << 1) | bits(SrcReg1, remaining - 1);
- quotient <<= 1;
- remaining--;
+ if (divisor & (ULL(1) << 63)) {
+ while (remaining && !(dividend & (ULL(1) << 63))) {
+ dividend = (dividend << 1) |
+ bits(SrcReg1, remaining - 1);
+ quotient <<= 1;
+ remaining--;
+ }
+ if (dividend & (ULL(1) << 63)) {
+ if (dividend < divisor && remaining) {
+ dividend = (dividend << 1) |
+ bits(SrcReg1, remaining - 1);
+ quotient <<= 1;
+ remaining--;
+ }
+ quotient++;
+ dividend -= divisor;
+ }
+ remainder = dividend;
+ } else {
+ //Shift in bits from the low order portion of the dividend
+ while (dividend < divisor && remaining) {
+ dividend = (dividend << 1) |
+ bits(SrcReg1, remaining - 1);
+ quotient <<= 1;
+ remaining--;
+ }
+ remainder = dividend;
+ //Do the division.
+ divide(dividend, divisor, quotient, remainder);
}
- remainder = dividend;
- //Do the division.
- divide(dividend, divisor, quotient, remainder);
}
//Keep track of how many bits there are still to pull in.
DestReg = merge(DestReg, remaining, dataSize);
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