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Add MULTIDRIVENPROC warning for signals driven by multiple plain always blocks (#7968)

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Aisha 2026-07-24 07:54:09 -05:00 committed by GitHub
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9 changed files with 181 additions and 14 deletions
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1
docs/CONTRIBUTORS
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@ -14,6 +14,7 @@ Adrian Sampson
Adrien Le Masle
أحمد المحمودي (Ahmed El-Mahmoudy)
Aidan McNay
Aisha Salimgereyeva
Aleksander Kiryk
Alex Chadwick
Alex Solomatnikov

6
docs/gen/ex_MULTIDRIVENPROC_faulty.rst Normal file
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@ -0,0 +1,6 @@
.. comment: generated by t_lint_multidriven_proc_bad
.. code-block:: sv
:linenos:
always @(posedge clk) q <= d;
always @(posedge clk) q <= ~d;

8
docs/gen/ex_MULTIDRIVENPROC_msg.rst Normal file
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@ -0,0 +1,8 @@
.. comment: generated by t_lint_multidriven_proc_bad
.. code-block::
%Warning-MULTIDRIVENPROC: example.v:1:25 Variable written to in always block also written by another always block: 'q'
: ... note: In instance 't'
example.v:1:25
16 | always @(posedge clk) q <= ~d;
| ^

26
docs/guide/warnings.rst
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@ -1474,6 +1474,32 @@ List Of Warnings
reduced optimizations.
.. option:: MULTIDRIVENPROC
Warns that the whole of a variable is driven by more than one plain
``always`` block. Unlike the :option:`MULTIDRIVEN` cases, plain
``always`` blocks carry no ``always_comb``/``always_ff`` intent, so this
is legal SystemVerilog rather than an IEEE 1800 violation. It is,
however, typically a synthesis error: hardware cannot have a signal
driven by two separate sequential blocks, so the design usually will not
behave as the RTL simulation suggests.
Disabled by default as this is a code-style warning; it will simulate
correctly.
Faulty example:
.. include:: ../../docs/gen/ex_MULTIDRIVENPROC_faulty.rst
Results in:
.. include:: ../../docs/gen/ex_MULTIDRIVENPROC_msg.rst
To fix, drive the signal from a single ``always`` block, or use
``always_ff``/``always_comb`` if the intent is a single specialized
process.
.. option:: MULTITOP
.. TODO better example

26
src/V3Error.h
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@ -135,6 +135,7 @@ public:
MODDUP, // Duplicate module
MODMISSING, // Error: missing module
MULTIDRIVEN, // Driven from multiple blocks
MULTIDRIVENPROC, // Driven from multiple plain always blocks
MULTITOP, // Multiple top level modules
NEWERSTD, // Newer language standard required
NOEFFECT, // Statement has no effect
@ -233,17 +234,17 @@ public:
"IEEEMAYDEPRECATE", "IFDEPTH", "IGNOREDRETURN", "IMPERFECTSCH", "IMPLICIT",
"IMPLICITSTATIC", "IMPORTSTAR", "IMPURE", "INCABSPATH", "INFINITELOOP", "INITIALDLY",
"INSECURE", "INSIDETRUE", "LATCH", "LITENDIAN", "MINTYPMAXDLY", "MISINDENT", "MODDUP",
"MODMISSING", "MULTIDRIVEN", "MULTITOP", "NEWERSTD", "NOEFFECT", "NOLATCH", "NONSTD",
"NORETURN", "NOTREDOP", "NULLPORT", "PARAMNODEFAULT", "PINCONNECTEMPTY", "PINMISSING",
"PINNOCONNECT", "PINNOTFOUND", "PKGNODECL", "PREPROCZERO", "PROCASSINIT",
"PROCASSWIRE", "PROFOUTOFDATE", "PROTECTED", "PROTOTYPEMIS", "RANDC", "REALCVT",
"REDEFMACRO", "RISEFALLDLY", "SELRANGE", "SHORTREAL", "SIDEEFFECT", "SPECIFYIGN",
"SPLITVAR", "STATICVAR", "STMTDLY", "SUPERNFIRST", "SYMRSVDWORD", "SYNCASYNCNET",
"TICKCOUNT", "TIMESCALEMOD", "UNDRIVEN", "UNOPT", "UNOPTFLAT", "UNOPTTHREADS",
"UNPACKED", "UNSATCONSTR", "UNSIGNED", "UNUSED", "UNUSEDGENVAR", "UNUSEDLOOP",
"UNUSEDPARAM", "UNUSEDSIGNAL", "USERERROR", "USERFATAL", "USERINFO", "USERWARN",
"VARHIDDEN", "WAITCONST", "WIDTH", "WIDTHCONCAT", "WIDTHEXPAND", "WIDTHTRUNC",
"WIDTHXZEXPAND", "ZERODLY", "ZEROREPL", " MAX"};
"MODMISSING", "MULTIDRIVEN", "MULTIDRIVENPROC", "MULTITOP", "NEWERSTD", "NOEFFECT",
"NOLATCH", "NONSTD", "NORETURN", "NOTREDOP", "NULLPORT", "PARAMNODEFAULT",
"PINCONNECTEMPTY", "PINMISSING", "PINNOCONNECT", "PINNOTFOUND", "PKGNODECL",
"PREPROCZERO", "PROCASSINIT", "PROCASSWIRE", "PROFOUTOFDATE", "PROTECTED",
"PROTOTYPEMIS", "RANDC", "REALCVT", "REDEFMACRO", "RISEFALLDLY", "SELRANGE",
"SHORTREAL", "SIDEEFFECT", "SPECIFYIGN", "SPLITVAR", "STATICVAR", "STMTDLY",
"SUPERNFIRST", "SYMRSVDWORD", "SYNCASYNCNET", "TICKCOUNT", "TIMESCALEMOD", "UNDRIVEN",
"UNOPT", "UNOPTFLAT", "UNOPTTHREADS", "UNPACKED", "UNSATCONSTR", "UNSIGNED", "UNUSED",
"UNUSEDGENVAR", "UNUSEDLOOP", "UNUSEDPARAM", "UNUSEDSIGNAL", "USERERROR", "USERFATAL",
"USERINFO", "USERWARN", "VARHIDDEN", "WAITCONST", "WIDTH", "WIDTHCONCAT",
"WIDTHEXPAND", "WIDTHTRUNC", "WIDTHXZEXPAND", "ZERODLY", "ZEROREPL", " MAX"};
return names[m_e];
}
// Warnings that default to off
@ -296,7 +297,8 @@ public:
return (m_e == ASSIGNDLY // More than style, but for backward compatibility
|| m_e == BLKSEQ || m_e == DECLFILENAME || m_e == DEFPARAM || m_e == EOFNEWLINE
|| m_e == GENUNNAMED || m_e == IMPORTSTAR || m_e == INCABSPATH
|| m_e == PINCONNECTEMPTY || m_e == PINNOCONNECT || m_e == PROCASSINIT
|| m_e == MULTIDRIVENPROC || m_e == PINCONNECTEMPTY || m_e == PINNOCONNECT
|| m_e == PROCASSINIT
|| m_e == SYNCASYNCNET || m_e == UNDRIVEN || m_e == UNUSEDGENVAR
|| m_e == UNUSEDLOOP || m_e == UNUSEDPARAM || m_e == UNUSEDSIGNAL
|| m_e == VARHIDDEN);

46
src/V3Undriven.cpp
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@ -47,6 +47,7 @@ class UndrivenVarEntry final {
const AstAlways* m_alwCombp
= nullptr; // always_comb of var if driven within always_comb, else nullptr
const AstAlways* m_alwFFp = nullptr; // always_ff of var if driven within always_ff
const AstAlways* m_alwPlainp = nullptr; // plain always of var if driven within plain always
const AstNodeVarRef* m_nodep = nullptr; // varref if driven, else nullptr
const AstNode* m_initStaticp = nullptr; // varref if in InitialStatic driven
const AstNode* m_initialp = nullptr; // varref if driven in an explicit initial block
@ -62,7 +63,8 @@ class UndrivenVarEntry final {
FLAG_DRIVEN = 1, // Signal or bit has been written/driven
FLAG_DRIVEN_ALWCOMB = 2, // Whole signal has been driven from always_comb
FLAG_DRIVEN_ALWFF = 3, // Whole signal has been driven from always_ff
FLAGS_PER_BIT = 4 // Number of flags stored for each tracked bit
FLAG_DRIVEN_ALWPLAIN = 4, // Whole signal has been driven from a plain always
FLAGS_PER_BIT = 5 // Number of flags stored for each tracked bit
};
public:
@ -158,6 +160,10 @@ public:
m_wholeFlags[FLAG_DRIVEN_ALWFF] = true;
m_alwFFp = alwFFp;
}
void drivenAlwaysPlainWhole(const AstAlways* alwPlainp) {
m_wholeFlags[FLAG_DRIVEN_ALWPLAIN] = true;
m_alwPlainp = alwPlainp;
}
const AstNode* initStaticp() const { return m_initStaticp; }
void initStaticp(const AstNode* nodep) { m_initStaticp = nodep; }
@ -172,10 +178,12 @@ public:
bool isDrivenWhole() const { return m_wholeFlags[FLAG_DRIVEN]; }
bool isDrivenAlwaysCombWhole() const { return m_wholeFlags[FLAG_DRIVEN_ALWCOMB]; }
bool isDrivenAlwaysFFWhole() const { return m_wholeFlags[FLAG_DRIVEN_ALWFF]; }
bool isDrivenAlwaysPlainWhole() const { return m_wholeFlags[FLAG_DRIVEN_ALWPLAIN]; }
bool isFtaskDriven() const { return m_ftaskDriven; }
const AstNodeVarRef* getNodep() const { return m_nodep; }
const AstAlways* getAlwCombp() const { return m_alwCombp; }
const AstAlways* getAlwFFp() const { return m_alwFFp; }
const AstAlways* getAlwPlainp() const { return m_alwPlainp; }
void usedBit(int bit, int width, const AstNode* nodep) {
UINFO(9, "set u[" << (bit + width - 1) << ":" << bit << "] " << m_varp->name());
for (int i = 0; i < width; i++) {
@ -375,6 +383,7 @@ class UndrivenVisitor final : public VNVisitorConst {
const AstAlways* m_alwaysp = nullptr; // Current always of either type
const AstAlways* m_alwaysCombp = nullptr; // Current always if combo, otherwise nullptr
const AstAlways* m_alwaysFFp = nullptr; // Current always if ff, otherwise nullptr
const AstAlways* m_alwaysPlainp = nullptr; // Current always if plain (not comb/ff/latch)
V3UndrivenCapture* const m_capturep = nullptr; // Capture object. 'nullptr' if disabled.
@ -536,7 +545,8 @@ class UndrivenVisitor final : public VNVisitorConst {
&& !VN_IS(nodep->dtypep()->skipRefp(), UnpackArrayDType) && !sameFileLine
&& !entryp->isUnderGen() && otherWritep && !entryp->isFtaskDriven()
&& !ftaskDef && !m_inSelLhs
&& !nodep->varp()->fileline()->warnIsOff(V3ErrorCode::MULTIDRIVEN)) {
&& (!nodep->varp()->fileline()->warnIsOff(V3ErrorCode::MULTIDRIVEN)
|| !nodep->varp()->fileline()->warnIsOff(V3ErrorCode::MULTIDRIVENPROC))) {
const bool otherWriteIsStaticInit
= nodep->varp()->hasUserInit() && otherWritep == entryp->initStaticp();
@ -587,6 +597,35 @@ class UndrivenVisitor final : public VNVisitorConst {
<< "... Location of always_ff write\n"
<< otherWritep->warnContextSecondary());
}
// Two plain always blocks driving the whole signal: legal
// SystemVerilog, but a driver conflict for synthesis. The
// always_ff/always_comb cases above already cover mixes with
// an explicit process, so only warn for plain+plain here.
// When the two blocks are clocked differently, the
// (on-by-default) MULTIDRIVEN check in V3Delayed already reports
// the conflict, so don't also emit MULTIDRIVENPROC for that case.
const AstAlways* const otherAlwaysp = entryp->getAlwPlainp();
const AstSenTree* const senp
= m_alwaysPlainp ? m_alwaysPlainp->sentreep() : nullptr;
const AstSenTree* const otherSenp
= otherAlwaysp ? otherAlwaysp->sentreep() : nullptr;
const bool differentClocking = senp && otherSenp && senp->hasClocked()
&& otherSenp->hasClocked()
&& !senp->sameTree(otherSenp);
if (m_alwaysPlainp && entryp->isDrivenAlwaysPlainWhole()
&& m_alwaysPlainp != otherAlwaysp
&& m_alwaysPlainp->fileline() != otherAlwaysp->fileline()
&& !differentClocking) {
nodep->v3warn(
MULTIDRIVENPROC,
"Variable written to in always block also written by another always "
"block: "
<< nodep->prettyNameQ() << '\n'
<< nodep->warnOther() << '\n'
<< nodep->warnContextPrimary() << '\n'
<< otherWritep->warnOther() << "... Location of other write\n"
<< otherWritep->warnContextSecondary());
}
}
if (!m_inInitialSetup || nodep->varp()->hasUserInit()) {
// Else don't count default initialization as a driver to a net/variable
@ -598,6 +637,7 @@ class UndrivenVisitor final : public VNVisitorConst {
entryp->underGenerate();
if (m_alwaysCombp) entryp->drivenAlwaysCombWhole(m_alwaysCombp);
if (m_alwaysFFp) entryp->drivenAlwaysFFWhole(m_alwaysFFp, nodep->varp());
if (m_alwaysPlainp) entryp->drivenAlwaysPlainWhole(m_alwaysPlainp);
}
if (nodep->access().isWriteOrRW() && !VN_IS(nodep, VarXRef)) {
// Ignoring xrefs as the initial and assignment to track might refer to two
@ -689,6 +729,7 @@ class UndrivenVisitor final : public VNVisitorConst {
VL_RESTORER(m_alwaysp);
VL_RESTORER(m_alwaysCombp);
VL_RESTORER(m_alwaysFFp);
VL_RESTORER(m_alwaysPlainp);
AstNode::user2ClearTree();
m_alwaysp = nodep;
if (nodep->keyword() == VAlwaysKwd::ALWAYS_COMB) {
@ -698,6 +739,7 @@ class UndrivenVisitor final : public VNVisitorConst {
m_alwaysCombp = nullptr;
}
m_alwaysFFp = nodep->keyword() == VAlwaysKwd::ALWAYS_FF ? nodep : nullptr;
m_alwaysPlainp = nodep->keyword() == VAlwaysKwd::ALWAYS ? nodep : nullptr;
iterateChildrenConst(nodep);
if (nodep->keyword() == VAlwaysKwd::ALWAYS_COMB) UINFO(9, " Done " << nodep);
}

20
test_regress/t/t_lint_multidriven_proc_bad.out Normal file
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@ -0,0 +1,20 @@
%Warning-MULTIDRIVENPROC: t/t_lint_multidriven_proc_bad.v:16:25: Variable written to in always block also written by another always block: 'q'
: ... note: In instance 't'
t/t_lint_multidriven_proc_bad.v:16:25:
16 | always @(posedge clk) q <= ~d;
| ^
t/t_lint_multidriven_proc_bad.v:15:25: ... Location of other write
15 | always @(posedge clk) q <= d;
| ^
... For warning description see https://verilator.org/warn/MULTIDRIVENPROC?v=latest
... Use "/* verilator lint_off MULTIDRIVENPROC */" and lint_on around source to disable this message.
%Warning-MULTIDRIVEN: t/t_lint_multidriven_proc_bad.v:26:18: Signal has multiple driving blocks with different clocking: 't2.q'
t/t_lint_multidriven_proc_bad.v:29:26: ... Location of first driving block
29 | always @(posedge clka) q <= d;
| ^
t/t_lint_multidriven_proc_bad.v:30:26: ... Location of other driving block
30 | always @(posedge clkb) q <= ~d;
| ^
... For warning description see https://verilator.org/warn/MULTIDRIVEN?v=latest
... Use "/* verilator lint_off MULTIDRIVEN */" and lint_on around source to disable this message.
%Error: Exiting due to

30
test_regress/t/t_lint_multidriven_proc_bad.py Normal file
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@ -0,0 +1,30 @@
#!/usr/bin/env python3
# DESCRIPTION: Verilator: Verilog Test driver/expect definition
#
# This program is free software; you can redistribute it and/or modify it
# under the terms of either the GNU Lesser General Public License Version 3
# or the Perl Artistic License Version 2.0.
# SPDX-FileCopyrightText: 2026 Wilson Snyder
# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
import vltest_bootstrap
test.scenarios('linter')
# MULTIDRIVENPROC is off by default; enable it explicitly. Module 't' (same
# clock) triggers it; module 't2' (different clocks) is reported by MULTIDRIVEN
# instead, confirming MULTIDRIVENPROC is suppressed there.
# -Wno-MULTITOP: both modules are top-level here on purpose (two independent
# demonstrations), so silence the unrelated multiple-top-module warning.
test.lint(fails=True, verilator_flags2=['-Wwarn-MULTIDRIVENPROC', '-Wno-MULTITOP'],
expect_filename=test.golden_filename)
test.extract(in_filename=test.top_filename,
out_filename=test.root + "/docs/gen/ex_MULTIDRIVENPROC_faulty.rst",
lines="15-16")
test.extract(in_filename=test.golden_filename,
out_filename=test.root + "/docs/gen/ex_MULTIDRIVENPROC_msg.rst",
lines="1-5")
test.passes()

32
test_regress/t/t_lint_multidriven_proc_bad.v Normal file
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@ -0,0 +1,32 @@
// DESCRIPTION: Verilator: Verilog Test module
//
// This file ONLY is placed under the Creative Commons Public Domain.
// SPDX-FileCopyrightText: 2026 Aisha Salimgereyeva
// SPDX-License-Identifier: CC0-1.0
// Same clock: two plain always blocks drive the whole of 'q', reported by
// MULTIDRIVENPROC.
module t (
input wire clk,
input wire d,
output logic q
);
always @(posedge clk) q <= d;
always @(posedge clk) q <= ~d;
endmodule
// Different clocks: reported by MULTIDRIVEN (on by default). MULTIDRIVENPROC is
// suppressed here so the conflict is reported only once.
module t2 (
input wire clka,
input wire clkb,
input wire d,
output logic q
);
always @(posedge clka) q <= d;
always @(posedge clkb) q <= ~d;
endmodule