Mario1159/verilator
1
0
Fork 0
forked from github/verilator
Code Pull Requests Releases 1 Activity

Simplify redundant masking of AstShiftR/AstShiftL

Browse Source 
AND(CONST,SHIFTR(_,C)) appears often after V3Expand, with C a large
enough dense mask (i.e.: of the form  (1 << n) - 1) to make the masking
redundant. E.g.: 0xff & ((uint32_t)a >> 24). V3Const now replaces these
ANDs with the SHIFTR node.

Similarly, we also simplify the same with SHIFTL,
e.g.: 0xff000000 & ((uint32_t)a << 24)
This commit is contained in:
Geza Lore 2021-06-18 18:52:02 +01:00
parent 0c93c3844f
commit 6c9c16c31d
1 changed files with 38 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

38
src/V3Const.cpp
View File

@ -752,6 +752,43 @@ private:
return false;
}
}
bool matchMaskedShift(AstAnd* nodep) {
// Drop redundant masking of right shift result. E.g: 0xff & ((uint32_t)a >> 24). This
// commonly appears after V3Expand and the simplification in matchMaskedOr. Similarly,
// drop redundant masking of left shift result. E.g.: 0xff000000 & ((uint32_t)a << 24).
if (nodep->width() != nodep->rhsp()->width()) return false; // Paranoia
const auto checkMask = [=](const V3Number& mask) -> bool {
const AstConst* const constp = VN_CAST(nodep->lhsp(), Const);
if (constp->num().isCaseEq(mask)) {
nodep->replaceWith(nodep->rhsp()->unlinkFrBack());
VL_DO_DANGLING(nodep->deleteTree(), nodep);
return true;
}
return false;
};
// Check if masking is redundant
if (AstShiftR* const shiftp = VN_CAST(nodep->rhsp(), ShiftR)) {
if (const AstConst* scp = VN_CAST_CONST(shiftp->rhsp(), Const)) {
// Check if mask is full over the non-zero bits
V3Number maskLo(nodep, nodep->width());
maskLo.setMask(nodep->width() - scp->num().toUInt());
return checkMask(maskLo);
}
} else if (AstShiftL* const shiftp = VN_CAST(nodep->rhsp(), ShiftL)) {
if (const AstConst* scp = VN_CAST_CONST(shiftp->rhsp(), Const)) {
// Check if mask is full over the non-zero bits
V3Number maskLo(nodep, nodep->width()), maskHi(nodep, nodep->width());
maskLo.setMask(nodep->width() - scp->num().toUInt());
maskHi.opShiftL(maskLo, scp->num());
return checkMask(maskHi);
}
}
return false;
}
bool matchBitOpTree(AstNode* nodep) {
if (!v3Global.opt.oConstBitOpTree()) return false;
@ -3008,6 +3045,7 @@ private:
// Common two-level operations that can be simplified
TREEOP ("AstAnd {$lhsp.castConst,matchAndCond(nodep)}", "DONE");
TREEOP ("AstAnd {$lhsp.castConst, $rhsp.castOr, matchMaskedOr(nodep)}", "DONE");
TREEOP ("AstAnd {$lhsp.castConst, matchMaskedShift(nodep)}", "DONE");
TREEOP ("AstAnd {$lhsp.castOr, $rhsp.castOr, operandAndOrSame(nodep)}", "replaceAndOr(nodep)");
TREEOP ("AstOr {$lhsp.castAnd,$rhsp.castAnd,operandAndOrSame(nodep)}", "replaceAndOr(nodep)");
TREEOP ("AstOr {matchOrAndNot(nodep)}", "DONE");