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Add comments in aarch64 branch functions

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Ajai Nelson 2023-04-09 14:33:43 -04:00
parent c71d7bb648
commit be3cf0abd3
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1 changed files with 10 additions and 0 deletions
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10
crates/compiler/gen_dev/src/generic64/aarch64.rs
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@ -1859,13 +1859,18 @@ fn asr_reg64_reg64_reg64(
/// `B.cond imm19` -> Jump to PC + imm19 if cond is met.
#[inline(always)]
fn b_cond_imm19(buf: &mut Vec<'_, u8>, cond: ConditionCode, imm19: i32) {
// Since instructions are 4 bytes, the branch instructions assume the last 2 bits are 0
debug_assert!(imm19 & 0b11 == 0, "branch location must be 4-byte aligned");
let shifted = imm19 >> 2;
let unsigned = shifted as u32;
// Our offset is only 19 bits, so we need to remove the first 13 bits
let left_removed = (unsigned << 13) >> 13;
// Check that imm19 wasn't too big
if imm19 >= 0 {
// Removing the first 13 bits should not have changed the value
debug_assert!(left_removed == unsigned);
} else {
// If imm19 was negative, left_removed will be sign-extended by the instruction
debug_assert!(left_removed | 0b1111_1111_1111_1100_0000_0000_0000_0000 == unsigned);
}
@ -1880,13 +1885,18 @@ fn b_cond_imm19(buf: &mut Vec<'_, u8>, cond: ConditionCode, imm19: i32) {
/// `B imm26` -> Jump to PC + imm26.
#[inline(always)]
fn b_imm26(buf: &mut Vec<'_, u8>, imm26: i32) {
// Since instructions are 4 bytes, the branch instructions assume the last 2 bits are 0
debug_assert!(imm26 & 0b11 == 0, "branch location must be 4-byte aligned");
let shifted = imm26 >> 2;
let unsigned = shifted as u32;
// Our offset is only 26 bits, so we need to remove the first 6 bits
let left_removed = (unsigned << 6) >> 6;
// Check that imm26 wasn't too big
if imm26 >= 0 {
// Removing the first 6 bits should not have changed the value
debug_assert!(left_removed == unsigned);
} else {
// If imm26 was negative, left_removed will be sign-extended by the instruction
debug_assert!(left_removed | 0b1111_1110_0000_0000_0000_0000_0000_0000 == unsigned);
}