A couple of IR simplification hacks for the amd64 front end, so as to

avoid false errors from memcheck.  Analogous to some of the recent
bunch of commits to x86 front end.

git-svn-id: svn://svn.valgrind.org/vex/trunk@1620

diff --git a/VEX/priv/guest-amd64/ghelpers.c b/VEX/priv/guest-amd64/ghelpers.c
index 8e8c707dde6be9f270c09aed66b522aaad1a470d..24c96ff227283c55635d8f2405985a2286e33dc1 100644 (file)
--- a/VEX/priv/guest-amd64/ghelpers.c
+++ b/VEX/priv/guest-amd64/ghelpers.c
@@ -1132,6 +1132,19 @@ IRExpr* guest_amd64_spechelper ( HChar* function_name,
                                         mkU64(0)));
       }
 
+      if (isU64(cc_op, AMD64G_CC_OP_LOGICB) && isU64(cond, AMD64CondS)) {
+         /* this is an idiom gcc sometimes uses to find out if the top
+            bit of a byte register is set: eg testb %al,%al; js ..
+            Since it just depends on the top bit of the byte, extract
+            that bit and explicitly get rid of all the rest.  This
+            helps memcheck avoid false positives in the case where any
+            of the other bits in the byte are undefined. */
+         /* byte and/or/xor, then S --> (UInt)result[7] */
+         return binop(Iop_And64,
+                      binop(Iop_Shr64,cc_dep1,mkU8(7)),
+                      mkU64(1));
+      }
+
       /*---------------- INCB ----------------*/
 
       if (isU64(cc_op, AMD64G_CC_OP_INCB) && isU64(cond, AMD64CondLE)) {

diff --git a/VEX/priv/ir/iropt.c b/VEX/priv/ir/iropt.c
index 94462d40218272a7f2563f2ca43d359e353b85e3..ea70bbda6ec0793d3fc64f19a7e0c318ff2f4c38 100644 (file)
--- a/VEX/priv/ir/iropt.c
+++ b/VEX/priv/ir/iropt.c
@@ -1419,6 +1419,17 @@ static IRExpr* fold_Expr ( IRExpr* e )
                               IRExpr_Const(IRConst_U8(1)));
          } else
 
+         /* Add64(t,t) ==> t << 1;  rationale as for Add32(t,t) above. */
+         if (e->Iex.Binop.op == Iop_Add64
+             && e->Iex.Binop.arg1->tag == Iex_Tmp
+             && e->Iex.Binop.arg2->tag == Iex_Tmp
+             && e->Iex.Binop.arg1->Iex.Tmp.tmp 
+                == e->Iex.Binop.arg2->Iex.Tmp.tmp) {
+            e2 = IRExpr_Binop(Iop_Shl64,
+                              e->Iex.Binop.arg1,
+                              IRExpr_Const(IRConst_U8(1)));
+         } else
+
          /* Or64/Add64(x,0) ==> x */
          if ((e->Iex.Binop.op == Iop_Add64 || e->Iex.Binop.op == Iop_Or64)
              && e->Iex.Binop.arg2->tag == Iex_Const