From 2770c141856eefda6a192622bf5fd5b06bef4963 Mon Sep 17 00:00:00 2001 From: Chandler Carruth Date: Mon, 14 Nov 2011 08:50:16 +0000 Subject: Fix an overflow bug in MachineBranchProbabilityInfo. This pass relied on the sum of the edge weights not overflowing uint32, and crashed when they did. This is generally safe as BranchProbabilityInfo tries to provide this guarantee. However, the CFG can get modified during codegen in a way that grows the *sum* of the edge weights. This doesn't seem unreasonable (imagine just adding more blocks all with the default weight of 16), but it is hard to come up with a case that actually triggers 32-bit overflow. Fortuately, the single-source GCC build is good at this. The solution isn't very pretty, but its no worse than the previous code. We're already summing all of the edge weights on each query, we can sum them, check for an overflow, compute a scale, and sum them again. I've included a *greatly* reduced test case out of the GCC source that triggers it. It's a pretty lame test, as it clearly is just barely triggering the overflow. I'd like to have something that is much more definitive, but I don't understand the fundamental pattern that triggers an explosion in the edge weight sums. The buggy code is duplicated within this file. I'll colapse them into a single implementation in a subsequent commit. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144526 91177308-0d34-0410-b5e6-96231b3b80d8 --- include/llvm/CodeGen/MachineBranchProbabilityInfo.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) (limited to 'include/llvm/CodeGen/MachineBranchProbabilityInfo.h') diff --git a/include/llvm/CodeGen/MachineBranchProbabilityInfo.h b/include/llvm/CodeGen/MachineBranchProbabilityInfo.h index 39d2f32982..943901f59f 100644 --- a/include/llvm/CodeGen/MachineBranchProbabilityInfo.h +++ b/include/llvm/CodeGen/MachineBranchProbabilityInfo.h @@ -34,8 +34,10 @@ class MachineBranchProbabilityInfo : public ImmutablePass { // weight to just "inherit" the non-zero weight of an adjacent successor. static const uint32_t DEFAULT_WEIGHT = 16; - // Get sum of the block successors' weights. - uint32_t getSumForBlock(MachineBasicBlock *MBB) const; + // Get sum of the block successors' weights, potentially scaling them to fit + // within 32-bits. If scaling is required, sets Scale based on the necessary + // adjustment. Any edge weights used with the sum should be divided by Scale. + uint32_t getSumForBlock(MachineBasicBlock *MBB, uint32_t &Scale) const; public: static char ID; -- cgit v1.2.3-18-g5258