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Refactor convert from float to unsigned int and add comments

pull/2440/head
Johnnie Birch 4 years ago
parent
ff8a4e4f9b
commit
09f3d4e331
1 changed files with 75 additions and 2 deletions
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  1. 77
      cranelift/codegen/src/isa/x64/lower.rs

77
cranelift/codegen/src/isa/x64/lower.rs
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@ -2722,6 +2722,7 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
} else {
if op == Opcode::FcvtToSintSat {
// Sets destination to zero if float is NaN
assert_eq!(types::F32X4, ctx.input_ty(insn, 0));
let tmp = ctx.alloc_tmp(RegClass::V128, types::I32X4);
ctx.emit(Inst::xmm_unary_rm_r(
SseOpcode::Movapd,
@ -2776,6 +2777,57 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
dst,
));
} else if op == Opcode::FcvtToUintSat {
// The algorithm for converting floats to unsigned ints is a little tricky. The
// complication arises because we are converting from a signed 64-bit int with a positive
// integer range from 1..INT_MAX (0x1..0x7FFFFFFF) to an unsigned integer with an extended
// range from (INT_MAX+1)..UINT_MAX. It's this range from (INT_MAX+1)..UINT_MAX
// (0x80000000..0xFFFFFFFF) that needs to be accounted for as a special case since our
// conversion instruction (cvttps2dq) only converts as high as INT_MAX (0x7FFFFFFF), but
// which conveniently setting underflows and overflows (smaller than MIN_INT or larger than
// MAX_INT) to be INT_MAX+1 (0x80000000). Nothing that the range (INT_MAX+1)..UINT_MAX includes
// precisely INT_MAX values we can correctly account for and convert every value in this range
// if we simply subtract INT_MAX+1 before doing the cvttps2dq conversion. After the subtraction
// every value originally (INT_MAX+1)..UINT_MAX is now the range (0..INT_MAX).
// After the conversion we add INT_MAX+1 back to this converted value, noting again that
// values we are trying to account for were already set to INT_MAX+1 during the original conversion.
// We simply have to create a mask and make sure we are adding together only the lanes that need
// to be accounted for. Digesting it all the steps then are:
//
// Step 1 - Account for NaN and negative floats by setting these src values to zero.
// Step 2 - Make a copy (tmp1) of the src value since we need to convert twice for
// reasons described above.
// Step 3 - Convert the original src values. This will convert properly all floats up to INT_MAX
// Step 4 - Subtract INT_MAX from the copy set (tmp1). Note, all zero and negative values are those
// values that were originally in the range (0..INT_MAX). This will come in handy during
// step 7 when we zero negative lanes.
// Step 5 - Create a bit mask for tmp1 that will correspond to all lanes originally less than
// UINT_MAX that are now less than INT_MAX thanks to the subtraction.
// Step 6 - Convert the second set of values (tmp1)
// Step 7 - Prep the converted second set by zeroing out negative lanes (these have already been
// converted correctly with the first set) and by setting overflow lanes to 0x7FFFFFFF
// as this will allow us to properly saturate overflow lanes when adding to 0x80000000
// Step 8 - Add the orginal converted src and the converted tmp1 where float values originally less
// than and equal to INT_MAX will be unchanged, float values originally between INT_MAX+1 and
// UINT_MAX will add together (INT_MAX) + (SRC - INT_MAX), and float values originally
// greater than UINT_MAX will be saturated to UINT_MAX (0xFFFFFFFF) after adding (0x8000000 + 0x7FFFFFFF).
//
//
// The table below illustrates the result after each step where it matters for the converted set.
// Note the original value range (original src set) is the final dst in Step 8:
//
// Original src set:
// | Original Value Range | Step 1 | Step 3 | Step 8 |
// | -FLT_MIN..FLT_MAX | 0.0..FLT_MAX | 0..INT_MAX(w/overflow) | 0..UINT_MAX(w/saturation) |
//
// Copied src set (tmp1):
// | Step 2 | Step 4 |
// | 0.0..FLT_MAX | (0.0-(INT_MAX+1))..(FLT_MAX-(INT_MAX+1)) |
//
// | Step 6 | Step 7 |
// | (0-(INT_MAX+1))..(UINT_MAX-(INT_MAX+1))(w/overflow) | ((INT_MAX+1)-(INT_MAX+1))..(INT_MAX+1) |
// Create temporaries
assert_eq!(types::F32X4, ctx.input_ty(insn, 0));
let tmp1 = ctx.alloc_tmp(RegClass::V128, types::I32X4);
let tmp2 = ctx.alloc_tmp(RegClass::V128, types::I32X4);
@ -2785,7 +2837,13 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
ctx.emit(Inst::gen_move(dst, src, input_ty));
ctx.emit(Inst::xmm_rm_r(SseOpcode::Maxps, RegMem::from(tmp2), dst));
// Set tmp2 to the maximum signed floating point value.
// Set tmp2 to INT_MAX+1. It is important to note here that after it looks
// like we are only converting INT_MAX (0x7FFFFFFF) but in fact because
// single precision IEEE-754 floats can only accurately represent contingous
// integers up to 2^23 and outside of this range it rounds to the closest
// integer that it can represent. In the case of INT_MAX, this value gets
// represented as 0x4f000000 which is the integer value (INT_MAX+1).
ctx.emit(Inst::xmm_rm_r(SseOpcode::Pcmpeqd, RegMem::from(tmp2), tmp2));
ctx.emit(Inst::xmm_rmi_reg(SseOpcode::Psrld, RegMemImm::imm(1), tmp2));
ctx.emit(Inst::xmm_rm_r(
@ -2794,8 +2852,17 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
tmp2,
));
// Make a copy of these lanes and then do the first conversion.
// Overflow lanes greater than the maximum allowed signed value will
// set to 0x80000000. Negative and NaN lanes will be 0x0
ctx.emit(Inst::xmm_mov(SseOpcode::Movaps, RegMem::from(dst), tmp1));
ctx.emit(Inst::xmm_rm_r(SseOpcode::Cvttps2dq, RegMem::from(dst), dst));
// Set lanes to src - max_signed_int
ctx.emit(Inst::xmm_rm_r(SseOpcode::Subps, RegMem::from(tmp2), tmp1));
// Create mask for all positive lanes to saturate (i.e. greater than
// or equal to the maxmimum allowable unsigned int).
let cond = FcmpImm::from(FloatCC::LessThanOrEqual);
ctx.emit(Inst::xmm_rm_r_imm(
SseOpcode::Cmpps,
@ -2805,16 +2872,22 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
false,
));
// Convert those set of lanes that have the max_signed_int factored out.
ctx.emit(Inst::xmm_rm_r(
SseOpcode::Cvttps2dq,
RegMem::from(tmp1),
tmp1,
));
// Prepare converted lanes by zeroing negative lanes and prepping lanes
// that have positive overflow (based on the mask) by setting these lanes
// to 0x7FFFFFFF
ctx.emit(Inst::xmm_rm_r(SseOpcode::Pxor, RegMem::from(tmp2), tmp1));
ctx.emit(Inst::xmm_rm_r(SseOpcode::Pxor, RegMem::from(tmp2), tmp2));
ctx.emit(Inst::xmm_rm_r(SseOpcode::Pmaxsd, RegMem::from(tmp2), tmp1));
ctx.emit(Inst::xmm_rm_r(SseOpcode::Cvttps2dq, RegMem::from(dst), dst));
// Add this second set of converted lanes to the original to properly handle
// values greater than max signed int.
ctx.emit(Inst::xmm_rm_r(SseOpcode::Paddd, RegMem::from(tmp1), dst));
} else {
// Since this branch is also guarded by a check for vector types

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