What are the ways compilers recognize complex patterns?
Compilers optimize by recognizing patterns like popcount, simplifying code for efficiency. LLVM and GCC use hardcoded patterns to match common idioms, balancing compile-time speed with runtime gains in critical code sections.
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The discussion on how compilers recognize complex patterns delves into the optimization process where compilers identify specific patterns in code to streamline operations. In a recent example, a compiler recognized a complex expression as a single operation, significantly simplifying the code. This optimization, implemented in LLVM and GCC, involves hardcoded patterns that match common coding idioms like popcount. While compilers do not simulate all possible inputs and outputs to detect these patterns, they employ techniques like pattern matching and canonicalization to streamline code. The use of hardcoded optimizations like popcount is a tradeoff between compile-time efficiency and runtime performance gains. Although compilers do not extensively rely on hardcoded patterns for optimizations, recognizing common idioms like popcount can lead to significant performance improvements, especially in critical code sections like inner loops. Overall, while compilers do not simulate all function inputs and outputs for optimizations, they use a mix of techniques to identify and streamline complex patterns in code efficiently.
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4 commentsCanonical forms are the really important part. Simple example: there are multiple ways a program might say “X * 2”. You could shift left by 1. You could multiple by 2. You could add X to itself. The idea of canonical forms is that in one pass, the compiler will pattern match all the ways you can do this and reduce all of them to the same canonical form - say, left shift by 1. Then, subsequent passes that want to catch more complex uses of that construct only have to look for one version of it (left shift 1) and not all three.
Here’s a more complex case. Ternary expressions in C and if-else statements have the same representation in llvm IR generated by clang: basic blocks and branches. There are multiple ways of representing the data flow (could use allocas and stores/loads or SSA data flow) but both the sroa and mem2reg passes will canonicalize to SSA. And, last I checked llvm says that the preferred canonical form of a if-then-else is a select (I.e. conditional move) whenever the two are equivalent. So, no matter what you use to write the equivalent of std::min - macros, templates, whatever, coding style don’t matter - you will end up eventually with a select instruction whose predicate is a comparison. Then - if your CPU supports doing min in a single instruction, it’s trivial for the instruction selector to just look for that kind of select. This happens not because every way of writing min is hardcoded, but because multiple rounds of canonicalization (clang using basic blocks and branches for both if/else and ternaries, sroa and mem2reg preferring SSA, and if conversion preferring select) gets you there.
A lot of this is hardcoding, but it’s not the boring “hardcode everything” kind of approach, but rather, it’s about using multiple phases that each produce increasingly canonical code that makes subsequent pattern matching simpler.
https://godbolt.org/z/8ronKz3Eb
Later an x86 backend can re-lower this into a popcnt instruction or to CPOP on a RISC-V backend or to CNT on ARMv8 or ….
https://godbolt.org/z/4zvWs6rzr
It can also be re-lowered to roughly those instructions on machines lacking population count instructions.
In this case: hardcoded search.
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