¶MapVector<llvm::Instruction*, uint64_t>
computeMinimumValueSizes(
    ArrayRef<llvm::BasicBlock*> Blocks,
    llvm::DemandedBits& DB,
    const llvm::TargetTransformInfo* TTI =
        nullptr)

Description

Compute a map of integer instructions to their minimum legal type size. C semantics force sub-int-sized values (e.g. i8, i16) to be promoted to int type (e.g. i32) whenever arithmetic is performed on them. For targets with native i8 or i16 operations, usually InstCombine can shrink the arithmetic type down again. However InstCombine refuses to create illegal types, so for targets without i8 or i16 registers, the lengthening and shrinking remains. Most SIMD ISAs (e.g. NEON) however support vectors of i8 or i16 even when their scalar equivalents do not, so during vectorization it is important to remove these lengthens and truncates when deciding the profitability of vectorization. This function analyzes the given range of instructions and determines the minimum type size each can be converted to. It attempts to remove or minimize type size changes across each def-use chain, so for example in the following code: %1 = load i8, i8* %2 = add i8 %1, 2 %3 = load i16, i16* %4 = zext i8 %2 to i32 %5 = zext i16 %3 to i32 %6 = add i32 %4, %5 %7 = trunc i32 %6 to i16 Instruction %6 must be done at least in i16, so computeMinimumValueSizes will return: {%1: 16, %2: 16, %3: 16, %4: 16, %5: 16, %6: 16, %7: 16}. If the optional TargetTransformInfo is provided, this function tries harder to do less work by only looking at illegal types.

Declared at: llvm/include/llvm/Analysis/VectorUtils.h:267

Parameters

ArrayRef<llvm::BasicBlock*> Blocks

llvm::DemandedBits& DB

const llvm::TargetTransformInfo* TTI = nullptr