Gabe Black has uploaded this change for review. (
https://gem5-review.googlesource.com/3381
Change subject: x86: Fix the multiplication microops.
......................................................................
x86: Fix the multiplication microops.
If the operands were 64 bit, an intermediate calculation could lose a
carry bit. This change rearranges that intermediate calculation if the
operand width is large, and reworks the microop implementation in general
in an attempt to make it easier to understand.
Change-Id: Ib36333f3f2695a33cd9623e43682de22ebd2e7ea
---
M src/arch/x86/isa/microops/regop.isa
1 file changed, 58 insertions(+), 22 deletions(-)
diff --git a/src/arch/x86/isa/microops/regop.isa
b/src/arch/x86/isa/microops/regop.isa
index ef0c4cb..dc5f0af 100644
--- a/src/arch/x86/isa/microops/regop.isa
+++ b/src/arch/x86/isa/microops/regop.isa
@@ -546,23 +546,42 @@
class Mul1s(WrRegOp):
op_class = 'IntMultOp'
+ # Multiply two values Aa and Bb where Aa = A << p + a, then
correct for
+ # negative operands.
+ # Aa * Bb
+ # = (A << p + a) * (B << p + b)
+ # = (A * B) << 2p + (A * b + a * B) << p + a * b
code = '''
ProdLow = psrc1 * op2;
- int halfSize = (dataSize * 8) / 2;
- uint64_t shifter = (ULL(1) << halfSize);
- uint64_t hiResult;
- uint64_t psrc1_h = psrc1 / shifter;
- uint64_t psrc1_l = psrc1 & mask(halfSize);
- uint64_t psrc2_h = (op2 / shifter) & mask(halfSize);
- uint64_t psrc2_l = op2 & mask(halfSize);
- hiResult = ((psrc1_l * psrc2_h + psrc1_h * psrc2_l +
- ((psrc1_l * psrc2_l) / shifter)) /shifter) +
- psrc1_h * psrc2_h;
+
+ int p = (dataSize * 8) / 2;
+ uint64_t A = bits(psrc1, 2 * p - 1, p);
+ uint64_t a = bits(psrc1, p - 1, 0);
+ uint64_t B = bits<uint64_t>(op2, 2 * p - 1, p);
+ uint64_t b = bits<uint64_t>(op2, p - 1, 0);
+
+ uint64_t c1, c2; // Carry between place values.
+ uint64_t ab = a * b, Ab = A * b, aB = a * B, AB = A * B;
+
+ c1 = ab >> p;
+
+ // Be careful to avoid overflow if p is large.
+ if (p == 32) {
+ c2 = (c1 >> 1) + (Ab >> 1) + (aB >> 1);
+ c2 += ((c1 & 0x1) + (Ab & 0x1) + (aB & 0x1)) >> 1;
+ c2 >>= (p - 1);
+ } else {
+ c2 = (c1 + Ab + aB) >> p;
+ }
+
+ uint64_t hi = AB + c2;
+
if (bits(psrc1, dataSize * 8 - 1))
- hiResult -= op2;
+ hi -= op2;
if (bits(op2, dataSize * 8 - 1))
- hiResult -= psrc1;
- ProdHi = hiResult;
+ hi -= psrc1;
+
+ ProdHi = hi;
'''
flag_code = '''
if ((-ProdHi & mask(dataSize * 8)) !=
@@ -578,17 +597,34 @@
class Mul1u(WrRegOp):
op_class = 'IntMultOp'
+ # Multiply two values Aa and Bb where Aa = A << p + a.
+ # Aa * Bb
+ # = (A << p + a) * (B << p + b)
+ # = (A * B) << 2p + (A * b + a * B) << p + a * b
code = '''
ProdLow = psrc1 * op2;
- int halfSize = (dataSize * 8) / 2;
- uint64_t shifter = (ULL(1) << halfSize);
- uint64_t psrc1_h = psrc1 / shifter;
- uint64_t psrc1_l = psrc1 & mask(halfSize);
- uint64_t psrc2_h = (op2 / shifter) & mask(halfSize);
- uint64_t psrc2_l = op2 & mask(halfSize);
- ProdHi = ((psrc1_l * psrc2_h + psrc1_h * psrc2_l +
- ((psrc1_l * psrc2_l) / shifter)) / shifter) +
- psrc1_h * psrc2_h;
+
+ int p = (dataSize * 8) / 2;
+ uint64_t A = bits(psrc1, 2 * p - 1, p);
+ uint64_t a = bits(psrc1, p - 1, 0);
+ uint64_t B = bits<uint64_t>(op2, 2 * p - 1, p);
+ uint64_t b = bits<uint64_t>(op2, p - 1, 0);
+
+ uint64_t c1, c2; // Carry between place values.
+ uint64_t ab = a * b, Ab = A * b, aB = a * B, AB = A * B;
+
+ c1 = ab >> p;
+
+ // Be careful to avoid overflow if p is large.
+ if (p == 32) {
+ c2 = (c1 >> 1) + (Ab >> 1) + (aB >> 1);
+ c2 += ((c1 & 0x1) + (Ab & 0x1) + (aB & 0x1)) >> 1;
+ c2 >>= (p - 1);
+ } else {
+ c2 = (c1 + Ab + aB) >> p;
+ }
+
+ ProdHi = AB + c2;
'''
flag_code = '''
if (ProdHi) {
--
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Gerrit-Project: public/gem5
Gerrit-Branch: master
Gerrit-MessageType: newchange
Gerrit-Change-Id: Ib36333f3f2695a33cd9623e43682de22ebd2e7ea
Gerrit-Change-Number: 3381
Gerrit-PatchSet: 1
Gerrit-Owner: Gabe Black <[email protected]>
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