From 45682fd6332c07f03608d6609836f7107f6a5a44 Mon Sep 17 00:00:00 2001 From: Florian Hahn Date: Thu, 28 Mar 2019 22:17:29 +0000 Subject: [LSR] Fix signed overflow in GenerateCrossUseConstantOffsets. For the attached test case, unchecked addition of immediate starts and ends overflows, as they can be arbitrary i64 constants. Proof: https://rise4fun.com/Alive/Plqc Reviewers: qcolombet, gilr, efriedma Reviewed By: efriedma Differential Revision: https://reviews.llvm.org/D59218 llvm-svn: 357217 --- llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) (limited to 'llvm/lib/Transforms') diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp index 340f5db4bdb..1cd28944350 100644 --- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp +++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp @@ -4133,11 +4133,17 @@ void LSRInstance::GenerateCrossUseConstantOffsets() { // Conservatively examine offsets between this orig reg a few selected // other orig regs. + int64_t First = Imms.begin()->first; + int64_t Last = std::prev(Imms.end())->first; + // Compute (First + Last) / 2 without overflow using the fact that + // First + Last = 2 * (First + Last) + (First ^ Last). + int64_t Avg = (First & Last) + ((First ^ Last) >> 1); + // If the result is negative and First is odd and Last even (or vice versa), + // we rounded towards -inf. Add 1 in that case, to round towards 0. + Avg = Avg + ((First ^ Last) & ((uint64_t)Avg >> 63)); ImmMapTy::const_iterator OtherImms[] = { - Imms.begin(), std::prev(Imms.end()), - Imms.lower_bound((Imms.begin()->first + std::prev(Imms.end())->first) / - 2) - }; + Imms.begin(), std::prev(Imms.end()), + Imms.lower_bound(Avg)}; for (size_t i = 0, e = array_lengthof(OtherImms); i != e; ++i) { ImmMapTy::const_iterator M = OtherImms[i]; if (M == J || M == JE) continue; -- cgit v1.2.3