diff options
| -rw-r--r-- | llvm/lib/CodeGen/IfConversion.cpp | 32 |
1 files changed, 31 insertions, 1 deletions
diff --git a/llvm/lib/CodeGen/IfConversion.cpp b/llvm/lib/CodeGen/IfConversion.cpp index 3fbab12a3a1..ba2fa4ce1f1 100644 --- a/llvm/lib/CodeGen/IfConversion.cpp +++ b/llvm/lib/CodeGen/IfConversion.cpp @@ -678,7 +678,37 @@ void IfConverter::ScanInstructions(BBInfo &BBI) { if (MI.isDebugValue()) continue; - if (MI.isNotDuplicable()) + // It's unsafe to duplicate convergent instructions in this context, so set + // BBI.CannotBeCopied to true if MI is convergent. To see why, consider the + // following CFG, which is subject to our "simple" transformation. + // + // BB0 // if (c1) goto BB1; else goto BB2; + // / \ + // BB1 | + // | BB2 // if (c2) goto TBB; else goto FBB; + // | / | + // | / | + // TBB | + // | | + // | FBB + // | + // exit + // + // Suppose we want to move TBB's contents up into BB1 and BB2 (in BB1 they'd + // be unconditional, and in BB2, they'd be predicated upon c2), and suppose + // TBB contains a convergent instruction. This is safe iff doing so does + // not add a control-flow dependency to the convergent instruction -- i.e., + // it's safe iff the set of control flows that leads us to the convergent + // instruction does not get smaller after the transformation. + // + // Originally we executed TBB if c1 || c2. After the transformation, there + // are two copies of TBB's instructions. We get to the first if c1, and we + // get to the second if !c1 && c2. + // + // There are clearly fewer ways to satisfy the condition "c1" than + // "c1 || c2". Since we've shrunk the set of control flows which lead to + // our convergent instruction, the transformation is unsafe. + if (MI.isNotDuplicable() || MI.isConvergent()) BBI.CannotBeCopied = true; bool isPredicated = TII->isPredicated(MI); |
