From 7197e2db28f10dec509967bb1cbd2d74cb03ee7e Mon Sep 17 00:00:00 2001 From: Yannick Moy Date: Tue, 17 Sep 2019 08:02:25 +0000 Subject: [PATCH] [Ada] Fix possible suppressed overflows in arithmetic run-time Function Double_Divide computes the division of its parameters (X / (Y*Z)) in a way that avoids overflows on signed integers, except in two specific cases, when X = -2**63, abs(Y) = abs(Z) = 1 (leading to an overflow in -To_Int(Qu)) and when X = -2**63 and Y*Z is large enough that Qu=0 and so the remainder Ru=2**63 (leading to an overflow in -To_Int(Ru)), for example with Y = Z = 2**32-1. This fix avoids the overflow by applying "-" on the unsigned value before the conversion to signed integer. The issue cannot manifest as an overflow check failure in our runtime, as overflow checks are suppressed by using pragma Suppress at the start of the file. Assuming a machine implements wraparound semantics here, the result was correct even with the suppressed overflow. As a result, there can be no test showing the difference. 2019-09-17 Yannick Moy gcc/ada/ * libgnat/s-arit64.adb (Double_Divide): Fix two possible overflows. From-SVN: r275790 --- gcc/ada/ChangeLog | 5 +++++ gcc/ada/libgnat/s-arit64.adb | 8 ++++++-- 2 files changed, 11 insertions(+), 2 deletions(-) diff --git a/gcc/ada/ChangeLog b/gcc/ada/ChangeLog index 13a3a766060..913b30fc700 100644 --- a/gcc/ada/ChangeLog +++ b/gcc/ada/ChangeLog @@ -1,3 +1,8 @@ +2019-09-17 Yannick Moy + + * libgnat/s-arit64.adb (Double_Divide): Fix two possible + overflows. + 2019-09-17 Dmitriy Anisimkov * make_util.ads (On_Windows): Move... diff --git a/gcc/ada/libgnat/s-arit64.adb b/gcc/ada/libgnat/s-arit64.adb index 3a65ec04d28..a35a40df5cf 100644 --- a/gcc/ada/libgnat/s-arit64.adb +++ b/gcc/ada/libgnat/s-arit64.adb @@ -204,9 +204,13 @@ package body System.Arith_64 is -- Case of dividend (X) sign negative + -- We perform the unary minus operation on the unsigned value + -- before conversion to signed, to avoid a possible overflow for + -- value -2**63, both for computing R and Q. + else - R := -To_Int (Ru); - Q := (if Den_Pos then -To_Int (Qu) else To_Int (Qu)); + R := To_Int (-Ru); + Q := (if Den_Pos then To_Int (-Qu) else To_Int (Qu)); end if; end Double_Divide; -- 2.30.2