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Add a comment with the BoolTest::cc2logical inferences tables, as
suggested by @tabjy.

Also, add a comment explaining how PhaseCCP::push_cmpu is handling
grandparent updates in the case 1b, as agreed with @chhagedorn.
This commit is contained in:
Francisco Ferrari Bihurriet 2025-08-14 07:47:08 +02:00
parent 27ed1a311e
commit 32a7940b8e
1 changed files with 35 additions and 2 deletions
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37
src/hotspot/share/opto/subnode.cpp
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@ -850,18 +850,51 @@ const Type *CmpUNode::sub( const Type *t1, const Type *t2 ) const {
// m > 0
// is false which is a contradiction.
//
// (1a) and (1b) is covered by this method since we can directly return the corresponding TypeInt::CC_*
// while (2) is covered in BoolNode::Ideal since we create a new non-constant node (see [CMPU_MASK]).
// (1a) and (1b) is covered by this method since we can directly return the
// corresponding TypeInt::CC_* while (2) is covered in BoolNode::Ideal since
// we create a new non-constant node (see [CMPU_MASK]).
//
// Depending on the _test of the child(ren) Bool node(s) of CmpU, the following
// optimizations will be later performed in BoolTest::cc2logical.
//
// (1a) "(x & m) <=u m" is always true, so type(CmpU) = CC_LE.
//
// | BoolTest | CmpU + Bool expression | Result |
// |----------|-------------------------|-------------------|
// | eq | (x & m) ==u m | unknown |
// | ne | (x & m) !=u m | unknown |
// |~~~ le ~~~|~~~~ (x & m) <=u m ~~~~|~~~~~~ true ~~~~~~~|
// | ge | (x & m) >=u m | unknown |
// | lt | (x & m) <u m | unknown |
// | gt | (x & m) >u m | false |
//
// (1b) "(x & m) <u m + 1" is always true (if m != -1), so type(CmpU) = CC_LT.
//
// | BoolTest | CmpU + Bool expression | Result if m != -1 |
// |----------|-------------------------|-------------------|
// | eq | (x & m) ==u m + 1 | false |
// | ne | (x & m) !=u m + 1 | true |
// | le | (x & m) <=u m + 1 | true |
// | ge | (x & m) >=u m + 1 | false |
// |~~~ lt ~~~|~~ (x & m) <u m + 1 ~~|~~~~~~ true ~~~~~~~|
// | gt | (x & m) >u m + 1 | false |
//
// NOTE: all the cases with "m & x" also apply.
//
const Type* CmpUNode::Value_cmpu_and_mask(PhaseValues* phase, const Node* andI, const Node* rhs) {
if (andI->Opcode() == Op_AndI) {
// (1a) "(x & m) <=u m" and "(m & x) <=u m" are always true,
// so CmpU(x & m, m) and CmpU(m & x, m) are known to be LE.
// NOTE: any CCP update to "m" will push the CmpUNode to the worklist,
// since rhs_m is a direct input of it.
const Node* rhs_m = rhs;
if (andI->in(2) == rhs_m || andI->in(1) == rhs_m) {
return TypeInt::CC_LE;
}
// (1b) "(x & m) <u m + 1" and "(m & x) <u m + 1" are always true for m != -1,
// so CmpU(x & m, m + 1) and CmpU(m & x, m + 1) are known to be LT.
// NOTE: any CCP update to "m" or "1" will push the CmpUNode to the worklist,
// this granchild push is handled by PhaseCCP::push_cmpu.
if (rhs->Opcode() == Op_AddI && rhs->in(2)->find_int_con(0) == 1) {
rhs_m = rhs->in(1);
const TypeInt* rhs_m_type = phase->type(rhs_m)->isa_int();