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When the input is a constant, we use a fairly inefficient sequence of fmov+fcvt+dup or, when the double isn't encodable in fmov, adr+ldr+fcvt+dup. Instead, we can use the same lowering as X64 when the input is a constant, and load the vector from memory. However, if the constant is encodable via fmov, we can use a vector fmov instead (which is just one instruction and doesn't need constant space). Fortunately the bit encoding of fmov for 32-bit floating point numbers matches that of 64-bit: the decoding algorithm is a little different because it expands into a larger exponent, but the values are compatible, so if a double can be encoded into a scalar fmov with a given abcdefgh pattern, the same pattern should encode the same float; due to the very limited number of mantissa and exponent bits, all values that are encodable are also exact in both 32-bit and 64-bit floats. This strategy is ~same as what gcc uses. For complex vectors, we previously used 4 instructions and 8 bytes of constant storage, and now we use 2 instructions and 16 bytes of constant storage, so the memory footprint is the same; for simple vectors we just need 1 instruction (4 bytes). clang lowers vector constants a little differently, opting to synthesize a 64-bit integer using 4 instructions (mov/movk) and then move it to the vector register - this requires 5 instructions and 20 bytes, vs ours/gcc 2 instructions and 8+16=24 bytes. I tried a simpler version of this that would be more compact - synthesize a 32-bit integer constant with mov+movk, and move it to vector register via dup.4s - but this was a little slower on M2, so for now we prefer the slightly larger version as it's not a regression vs current implementation. |
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| .. | ||
| AddressA64.h | ||
| AssemblyBuilderA64.h | ||
| AssemblyBuilderX64.h | ||
| BytecodeAnalysis.h | ||
| BytecodeSummary.h | ||
| CodeAllocator.h | ||
| CodeBlockUnwind.h | ||
| CodeGen.h | ||
| CodeGenCommon.h | ||
| ConditionA64.h | ||
| ConditionX64.h | ||
| IrAnalysis.h | ||
| IrBuilder.h | ||
| IrCallWrapperX64.h | ||
| IrData.h | ||
| IrDump.h | ||
| IrRegAllocX64.h | ||
| IrUtils.h | ||
| IrVisitUseDef.h | ||
| Label.h | ||
| OperandX64.h | ||
| OptimizeConstProp.h | ||
| OptimizeDeadStore.h | ||
| OptimizeFinalX64.h | ||
| RegisterA64.h | ||
| RegisterX64.h | ||
| UnwindBuilder.h | ||
| UnwindBuilderDwarf2.h | ||
| UnwindBuilderWin.h | ||