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1. Addition of !noundef Metadata to Load Instructions
Several load instructions now include the !noundef metadata, which indicates that the loaded value is guaranteed not to be undefined. This change enhances optimization opportunities by allowing the compiler to assume that the loaded value is always valid and does not require additional checks for undefined behavior. For example:
2. Update of Branch Conditions with !llvm.loop Metadata
Branch conditions in loops have been updated to include !llvm.loop metadata, which provides loop-specific information to the optimizer. This can help the compiler perform more aggressive loop optimizations, such as unrolling or vectorization. For instance:
The freeze intrinsic has been removed from several instructions, simplifying the code and reducing unnecessary operations. The freeze intrinsic was used to ensure that a value is not undefined, but with the addition of !noundef, it is no longer needed. Example:
Previously, this would have included a freeze operation on %pre16.i.
4. Alignment of Metadata Indices
Some metadata indices have been aligned or reordered to improve consistency and clarity. For example, the alignment of !tbaa (Type-Based Alias Analysis) and !range metadata ensures that the optimizer has better information about memory access patterns. Example:
In some cases, complex conditional logic involving multiple select or and operations has been simplified. For example, instead of using select to combine conditions, an and operation is now used:
%or.cond.i.i.i = andi1%99, %101
This simplification reduces instruction complexity and improves readability while maintaining equivalent functionality.
High-Level Overview
The patch primarily focuses on enhancing the safety and optimization potential of the LLVM IR by:
Adding !noundef metadata to load instructions, ensuring that values are always defined.
Updating branch conditions with !llvm.loop metadata to enable advanced loop optimizations.
Removing redundant freeze intrinsics, streamlining the IR.
Aligning and reordering metadata indices for improved alias analysis and range information.
Simplifying conditional logic to reduce complexity and improve performance.
These changes collectively aim to improve the generated code's efficiency and reliability, leveraging modern LLVM optimization techniques while adhering to stricter safety guarantees.
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Link: llvm/llvm-project#134403
Requested by: @dtcxzyw