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commit d6a810760ec61ddedf15445457edbbe288536a2f
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Author: Julian Seward <jseward@acm.org>
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Date: Tue Dec 12 22:31:54 2017 +0100
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Fix false positive with s390x cgijnl instruction testing against sign bit.
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https://bugs.kde.org/show_bug.cgi?id=387712
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When the cgij "compare immediate and branch relative" instruction
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compares 0 <=signed dep1, that means dep1 >=signed 0, so it is a test
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against the most significant bit of dep1. So only that bit needs
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to be defined.
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diff --git a/VEX/priv/guest_s390_helpers.c b/VEX/priv/guest_s390_helpers.c
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index 4cccdec..aacd833 100644
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--- a/VEX/priv/guest_s390_helpers.c
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+++ b/VEX/priv/guest_s390_helpers.c
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@@ -1818,6 +1818,13 @@ isC64(const IRExpr *expr)
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return expr->tag == Iex_Const && expr->Iex.Const.con->tag == Ico_U64;
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}
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+static inline Bool
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+isC64_exactly(const IRExpr *expr, ULong n)
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+{
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+ return expr->tag == Iex_Const && expr->Iex.Const.con->tag == Ico_U64
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+ && expr->Iex.Const.con->Ico.U64 == n;
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+}
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+
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/* The returned expression is NULL if no specialization was found. In that
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case the helper function will be called. Otherwise, the expression has
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@@ -1895,9 +1902,25 @@ guest_s390x_spechelper(const HChar *function_name, IRExpr **args,
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}
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/* cc_dep1 > cc_dep2 ----> cc_dep2 < cc_dep1 */
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if (cond == 2 || cond == 2 + 1) {
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+ /* If we ever need the counterpart of the bug387712 fix just
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+ below, then here is the place. We'll need to give an
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+ alternative expression for the case "cc_dep2
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+ bit of simple testing, I've yet to see any such cases,
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+ however. */
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return unop(Iop_1Uto32, binop(Iop_CmpLT64S, cc_dep2, cc_dep1));
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}
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if (cond == 8 + 2 || cond == 8 + 2 + 1) {
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+ if (isC64_exactly(cc_dep2, 0)) {
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+ /* 0 <=signed dep1
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+ --> dep1 >=signed 0
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+ --> m.s.bit of dep1 == 0 */
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+ /* See bug 387712. This is an old trick from gcc to extract
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+ the most significant bit of a word. */
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+ return unop(Iop_64to32,
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+ binop(Iop_Xor64,
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+ binop(Iop_Shr64, cc_dep1, mkU8(63)),
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+ mkU64(1)));
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+ }
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return unop(Iop_1Uto32, binop(Iop_CmpLE64S, cc_dep2, cc_dep1));
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}
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if (cond == 8 + 4 + 2 || cond == 8 + 4 + 2 + 1) {
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