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better comments for the DIV functions

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Folkert 2022-08-23 10:29:04 +02:00
parent 4f84a120c5
commit 8305d3b037
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1 changed files with 14 additions and 5 deletions
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19
crates/compiler/gen_dev/src/generic64/x86_64.rs
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@ -1695,7 +1695,7 @@ fn mul_reg64_reg64(buf: &mut Vec<'_, u8>, src: X86_64GeneralReg) {
buf.extend(&[rex, 0xF7, 0b1110_0000 | (src as u8 % 8)]);
}
/// `IDIV r/m64` -> Signed Divide r/m64 to r64.
/// `IDIV r/m64` -> Signed divide RDX:RAX by r/m64, with result stored in RAX ← Quotient, RDX ← Remainder.
#[inline(always)]
fn idiv_reg64_reg64(buf: &mut Vec<'_, u8>, src: X86_64GeneralReg) {
let mut rex = REX_W;
@ -1705,11 +1705,17 @@ fn idiv_reg64_reg64(buf: &mut Vec<'_, u8>, src: X86_64GeneralReg) {
rex |= REX_PREFIX_B;
}
// adds a cqo (convert doubleword to quadword)
buf.extend(&[0x48, 0x99, rex, 0xF7, 0b1111_1000 | (src as u8 % 8)]);
// The CQO instruction can be used to produce a double quadword dividend
// from a quadword before a quadword division.
//
// The CQO instruction (available in 64-bit mode only) copies the sign (bit 63)
// of the value in the RAX register into every bit position in the RDX register
buf.extend(&[0x48, 0x99]);
buf.extend(&[rex, 0xF7, 0b1111_1000 | (src as u8 % 8)]);
}
/// `DIV r/m64` -> Unsigned Divide r/m64 to r64.
/// `DIV r/m64` -> Unsigned divide RDX:RAX by r/m64, with result stored in RAX ← Quotient, RDX ← Remainder.
#[inline(always)]
fn udiv_reg64_reg64(buf: &mut Vec<'_, u8>, src: X86_64GeneralReg) {
let mut rex = REX_W;
@ -1719,8 +1725,11 @@ fn udiv_reg64_reg64(buf: &mut Vec<'_, u8>, src: X86_64GeneralReg) {
rex |= REX_PREFIX_B;
}
// `xor edx, edx`, clears the edx register
buf.extend(&[0x31, 0b1101_0010]);
// adds a cqo (convert doubleword to quadword)
buf.extend(&[0x48, 0x99, rex, 0xF7, 0b1111_0000 | (src as u8 % 8)]);
buf.extend(&[rex, 0xF7, 0b1111_0000 | (src as u8 % 8)]);
}
/// Jump near, relative, RIP = RIP + 32-bit displacement sign extended to 64-bits.