fix for signed comparison bug

google · Feb 4, 2022 · d58a45d · d58a45d
1 parent d0fd66c
commit d58a45d
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Showing 2 changed files with 9 additions and 5 deletions.
diff --git a/proj/hps_accel/gateware/gen2/hps_cfu.py b/proj/hps_accel/gateware/gen2/hps_cfu.py
@@ -205,11 +205,12 @@ def max_(word0, word1):
             bytes0 = [word0[i:i + 8] for i in range(0, 32, 8)]
             bytes1 = [word1[i:i + 8] for i in range(0, 32, 8)]
             for r, b0, b1 in zip(result, bytes0, bytes1):
-                sb0 = Signal(signed(8))
-                m.d.comb += sb0.eq(b0)
-                sb1 = Signal(signed(8))
-                m.d.comb += sb1.eq(b1)
-                m.d.comb += r.eq(Mux(sb1 > sb0, b1, b0))
+                # the bytes represent signed values, but signed comparison
+                # appears to trigger a bug
+                # https://github.com/google/CFU-Playground/issues/451
+                # To avoid that bug, we flip the sign bits (i.e add 128)
+                # and do an unsigned comparison instead.
+                m.d.comb += r.eq(Mux((b1 ^ 0x80) > (b0 ^ 0x80), b1, b0))
             return Cat(*result)
 
         last2 = Signal(32)
@@ -225,6 +226,7 @@ def max_(word0, word1):
 class HpsCfu(Cfu):
     """Gen2 accelerator CFU.
     """
+
     def __init__(self, specialize_nx=False):
         """Constructor
 

diff --git a/proj/hps_accel/gateware/gen2/test_hps_cfu.py b/proj/hps_accel/gateware/gen2/test_hps_cfu.py
@@ -238,6 +238,8 @@ def test_pool(self):
             ((Constants.INS_POOL, 0x80808080, 0x80808080), 0x80808080),
             ((Constants.INS_POOL, 0x08030601, 0x04070205), 0x08070605),
             ((Constants.INS_POOL, 0x00660000, 0x00007700), 0x08667705),
+            ((Constants.INS_POOL, 0x12808080, 0x88808080), None),
+            ((Constants.INS_POOL, 0x1a808080, 0x81808080), 0x1a808080),
         ]
 
         self.run_ops(OPS, False)