From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Mon, 18 Apr 2022 19:33:31 +0000 (+0100)
Subject: experiment expressing borrow as ~val+1 instead of -val
X-Git-Tag: opf_rfc_ls005_v1~2724
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=bd4cfa18b0ed7828de9bc9ae2b8ef9542f2bbe9c;p=libreriscv.git

experiment expressing borrow as ~val+1 instead of -val
---

diff --git a/openpower/sv/bitmanip/divmnu64.c b/openpower/sv/bitmanip/divmnu64.c
index 52f1715aa..bc9b033b1 100644
--- a/openpower/sv/bitmanip/divmnu64.c
+++ b/openpower/sv/bitmanip/divmnu64.c
@@ -107,7 +107,7 @@ again:
         if (rhat < b) goto again;
       }
 
-#define MUL_RSUB_CARRY1
+#define SUB_MUL_BORROW
 #ifdef ORIGINAL
       // Multiply and subtract.
       k = 0;
@@ -141,7 +141,7 @@ again:
       // this is the new (odd) instruction
       for(int i = 0; i <= n; i++) {
          uint64_t value = (((uint64_t)phi[i]<<32) | plo[i]) - borrow;
-         borrow = -(uint32_t)(value >> 32);
+         borrow = ~(value >> 32)+1; // -(uint32_t)(value >> 32);
          un[i + j] = (uint32_t)value;
       }
       bool need_fixup = borrow != 0;