diff options
| -rw-r--r-- | lib/Target/X86/X86FastISel.cpp | 23 |
1 files changed, 23 insertions, 0 deletions
diff --git a/lib/Target/X86/X86FastISel.cpp b/lib/Target/X86/X86FastISel.cpp index 6bbc984..7984e76 100644 --- a/lib/Target/X86/X86FastISel.cpp +++ b/lib/Target/X86/X86FastISel.cpp @@ -633,6 +633,29 @@ bool X86FastISel::X86SelectCallAddress(const Value *V, X86AddressMode &AM) { const User *U = NULL; unsigned Opcode = Instruction::UserOp1; const Instruction *I = dyn_cast<Instruction>(V); + // Record if the value is defined in the same basic block. + // + // This information is crucial to know whether or not folding an + // operand is valid. + // Indeed, FastISel generates or reuses a virtual register for all + // operands of all instructions it selects. Obviously, the definition and + // its uses must use the same virtual register otherwise the produced + // code is incorrect. + // Before instruction selection, FunctionLoweringInfo::set sets the virtual + // registers for values that are alive across basic blocks. This ensures + // that the values are consistently set between across basic block, even + // if different instruction selection mechanisms are used (e.g., a mix of + // SDISel and FastISel). + // For values local to a basic block, the instruction selection process + // generates these virtual registers with whatever method is appropriate + // for its needs. In particular, FastISel and SDISel do not share the way + // local virtual registers are set. + // Therefore, this is impossible (or at least unsafe) to share values + // between basic blocks unless they use the same instruction selection + // method, which is not guarantee for X86. + // Moreover, things like hasOneUse could not be used accurately, if we + // allow to reference values across basic blocks whereas they are not + // alive across basic blocks initially. bool InMBB = true; if (I) { Opcode = I->getOpcode(); |