Patch pre-ISAv2 y-bit handling for branch prediction modifiers

2026-07-10 03:18:49 -07:00 · 2026-02-20 10:35:11 -07:00
parent d1b276a437
commit 8214ce3d38
1 changed files with 57 additions and 0 deletions
@@ -0,0 +1,57 @@
+--- a/opcodes/ppc-opc.c	2026-02-20 10:07:16.238782135 -0700
+++ b/opcodes/ppc-opc.c	2026-02-20 10:02:17.768706877 -0700
+@@ -248,7 +248,13 @@
+ {
+   if ((dialect & ISA_V2) == 0)
+     {
+-      if (((insn & (1 << 21)) == 0) != ((insn & (1 << 15)) == 0))
+      /* For pre-ISA_V2 (y-bit scheme), the "-" suffix should only be
+	 shown when the prediction differs from the default.  The y-bit
+	 (bit 21) reverses the default static prediction: backward
+	 branches (negative BD, bit 15 set) are predicted taken, forward
+	 branches are predicted not-taken.  So "-" (predict not-taken)
+	 is non-default only when y=1 AND the branch is backward.  */
+      if ((insn & (1 << 21)) == 0 || (insn & (1 << 15)) == 0)
+ 	*invalid = 1;
+     }
+   else
+@@ -293,7 +299,13 @@
+ {
+   if ((dialect & ISA_V2) == 0)
+     {
+-      if (((insn & (1 << 21)) == 0) == ((insn & (1 << 15)) == 0))
+      /* For pre-ISA_V2 (y-bit scheme), the "+" suffix should only be
+	 shown when the prediction differs from the default.  The y-bit
+	 (bit 21) reverses the default static prediction: backward
+	 branches (negative BD, bit 15 set) are predicted taken, forward
+	 branches are predicted not-taken.  So "+" (predict taken) is
+	 non-default only when y=1 AND the branch is forward.  */
+      if ((insn & (1 << 21)) == 0 || (insn & (1 << 15)) != 0)
+ 	*invalid = 1;
+     }
+   else
+--- a/opcodes/ppc-dis.c	2024-01-28 17:00:00.000000000 -0700
+++ b/opcodes/ppc-dis.c	2026-02-20 10:03:40.457706897 -0700
+@@ -639,6 +639,22 @@
+ 	  || (opcode->deprecated & dialect & PPC_OPCODE_RAW) != 0)
+ 	continue;
+ 
+      /* For pre-ISA_V2 (y-bit scheme), XL-form branch instructions
+	 (bclr, bcctr) have a default prediction of "not taken".
+	 The y-bit (bit 21) reverses this to "taken".  Since "not taken"
+	 is always the default, the "-" suffix is never meaningful --
+	 it would be identical to the unsuffixed form.  Skip "-" entries
+	 so the unsuffixed form matches instead.  */
+      if ((dialect & (PPC_OPCODE_POWER4
+		      | PPC_OPCODE_E500MC
+		      | PPC_OPCODE_TITAN)) == 0
+	  && op == 19)
+	{
+	  const char *name = opcode->name;
+	  if (name[strlen (name) - 1] == '-')
+	    continue;
+	}
+
+       /* Check validity of operands.  */
+       invalid = 0;
+       for (opindex = opcode->operands; *opindex != 0; opindex++)