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(* M2SSA.mod discover very obvious single assignment temporaries.
Copyright (C) 2021-2025 Free Software Foundation, Inc.
Contributed by Gaius Mulley <gaius.mulley@southwales.ac.uk>.
This file is part of GNU Modula-2.
GNU Modula-2 is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3, or (at your option)
any later version.
GNU Modula-2 is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with GNU Modula-2; see the file COPYING3. If not see
<http://www.gnu.org/licenses/>. *)
IMPLEMENTATION MODULE M2SSA ;
FROM M2Debug IMPORT Assert ;
FROM NameKey IMPORT Name, WriteKey, MakeKey, GetKey ;
FROM StrIO IMPORT WriteString, WriteLn ;
FROM NumberIO IMPORT WriteCard ;
FROM M2Error IMPORT InternalError ;
FROM M2Batch IMPORT GetModuleNo ;
FROM M2Quiet IMPORT qprintf1 ;
FROM M2Scope IMPORT ScopeBlock, InitScopeBlock, KillScopeBlock ;
FROM M2StackWord IMPORT StackOfWord, InitStackWord, KillStackWord, PushWord, PopWord, PeepWord ;
FROM M2Options IMPORT CompilerDebugging ;
FROM Lists IMPORT InitList, KillList, List, IncludeItemIntoList, IsItemInList ;
FROM M2Printf IMPORT printf0, printf1, printf2 ;
FROM SymbolTable IMPORT GetSymName,
GetProcedureQuads, GetModuleQuads,
GetModule, GetNthProcedure,
GetSubrange, GetModuleScope,
PutProcedureReachable, IsProcedureReachable,
PutProcedureStartQuad, PutProcedureEndQuad,
PutProcedureScopeQuad,
PutNeedSavePriority,
IsProcedure, GetPriority,
GetDeclaredMod, GetFirstUsed,
GetType, GetNth, GetWriteQuads, GetReadQuads, GetMode,
IsExportQualified, IsExportUnQualified, IsExported,
ForeachProcedureDo, ForeachInnerModuleDo,
IsModuleWithinProcedure, IsTemporary,
PutVariableSSA,
NulSym ;
FROM M2Quads IMPORT QuadOperator, GetQuad, GetFirstQuad, GetNextQuad,
PutQuad, SubQuad, Opposite, IsReferenced,
GetRealQuad ;
VAR
stack : StackOfWord ;
visited: List ;
(*
writtenOnce - return TRUE if variable is written to once.
*)
PROCEDURE writtenOnce (variable: CARDINAL) : BOOLEAN ;
VAR
writeStart, writeEnd: CARDINAL ;
readStart, readEnd : CARDINAL ;
BEGIN
GetWriteQuads (variable, GetMode (variable), writeStart, writeEnd) ;
GetReadQuads (variable, GetMode (variable), readStart, readEnd) ;
RETURN (writeStart = writeEnd) AND ((readStart > writeStart) OR (readStart = 0))
END writtenOnce ;
(*
DetermineSSA - performs a trivial check to see if the temporary is written to
once.
*)
PROCEDURE DetermineSSA (variable: CARDINAL) ;
VAR
name: Name ;
BEGIN
IF EnableSSA AND IsTemporary (variable)
THEN
name := GetSymName (variable) ;
IF writtenOnce (variable)
THEN
PutVariableSSA (variable, TRUE) ;
IF CompilerDebugging
THEN
printf1 (" temporary: %a SSA found\n", name)
END
ELSE
IF CompilerDebugging
THEN
printf1 (" temporary: %a not SSA\n", name)
END
END
END
END DetermineSSA ;
(*
DiscoverSSATemporaries -
*)
PROCEDURE DiscoverSSATemporaries (scope: CARDINAL) ;
VAR
n,
variable: CARDINAL ;
BEGIN
IF CompilerDebugging
THEN
printf1 ("examining scope %d\n", scope)
END ;
n := 1 ;
variable := GetNth (scope, n) ;
WHILE variable # NulSym DO
DetermineSSA (variable) ;
INC (n) ;
variable := GetNth (scope, n)
END
END DiscoverSSATemporaries ;
(*
DiscoverSSA - perform a very simple check to determine whether a
temporary is a single use write.
*)
PROCEDURE DiscoverSSA (scope: CARDINAL) ;
VAR
sb: ScopeBlock ;
BEGIN
IF NOT IsItemInList (visited, scope)
THEN
IncludeItemIntoList (visited, scope) ;
sb := InitScopeBlock (scope) ;
PushWord (stack, scope) ;
IF CompilerDebugging
THEN
printf1 ("DiscoverSSA %d\n", scope)
END ;
IF CompilerDebugging
THEN
printf0 ("ForeachInnerModuleDo\n")
END ;
ForeachInnerModuleDo(scope, DiscoverSSA) ;
IF CompilerDebugging
THEN
printf0 ("ForeachProcedureDo\n")
END ;
ForeachProcedureDo(scope, DiscoverSSA) ;
DiscoverSSATemporaries (scope) ;
Assert (PopWord (stack) = scope) ;
KillScopeBlock (sb)
END
END DiscoverSSA ;
BEGIN
stack := InitStackWord () ;
InitList (visited)
END M2SSA.
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