From: Konstantinos Margaritis <konstantinos.margaritis@vectorcamp.gr>
Date: Fri, 28 Apr 2023 16:41:22 +0000 (+0000)
Subject: handle negatives correctly by adding sign bit to final result
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=426f6908e4cb47e7b8c29318868bdde7139134ef;p=openpower-isa.git

handle negatives correctly by adding sign bit to final result
---

diff --git a/openpower/isa/butterfly.mdwn b/openpower/isa/butterfly.mdwn
index 277305a8..555e8abd 100644
--- a/openpower/isa/butterfly.mdwn
+++ b/openpower/isa/butterfly.mdwn
@@ -18,12 +18,14 @@ Pseudo-code:
     res1 <- ROTL64(prod1, XLEN-n)
     res2 <- ROTL64(prod2, XLEN-n)
     m <- MASK(n, (XLEN-1))
-    s1 <- res1[0]
-    s2 <- res2[0]
-    smask1 <- ([s1]*XLEN) & ¬m
-    smask2 <- ([s2]*XLEN) & ¬m
-    RT <- res1 & m | smask1
-    RS <- res2 & m | smask2
+    signbit1 <- res1[0]
+    signbit2 <- res2[0]
+    smask1 <- ([signbit1]*XLEN) & ¬m
+    smask2 <- ([signbit2]*XLEN) & ¬m
+    s64_1 <- [0]*(XLEN-1) || signbit1
+    s64_2 <- [0]*(XLEN-1) || signbit2
+    RT <- (res1 & m | smask1) + s64_1
+    RS <- (res2 & m | smask2) + s64_2
 
 Special Registers Altered:
 
diff --git a/src/openpower/test/alu/maddsubrs_cases.py b/src/openpower/test/alu/maddsubrs_cases.py
index 9dd48c97..feace48d 100644
--- a/src/openpower/test/alu/maddsubrs_cases.py
+++ b/src/openpower/test/alu/maddsubrs_cases.py
@@ -24,5 +24,6 @@ class MADDSUBRSTestCase(TestAccumulatorBase):
         e = ExpectedState(pc=4)
         e.intregs[1] = 0x0000aa85
         e.intregs[2] = 0xffffffffffff643e
+        e.intregs[3] = 0x00002d41
         self.add_case(Program(lst, bigendian), initial_regs, expected=e)