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Handle correctly general switch statements when simplifying #3624

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Nov 2, 2022
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Handle correctly general switch statements when simplifying #3624

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b5980b6
Handle correctly general switch statements when simplifying
Oct 26, 2022
75bd26b
Properly handle side-effects when removing switch statements
Oct 28, 2022
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24 changes: 14 additions & 10 deletions frontends/p4/simplify.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@ limitations under the License.

#include "simplify.h"
#include "sideEffects.h"
#include "tableApply.h"

namespace P4 {

Expand Down Expand Up @@ -92,16 +93,19 @@ const IR::Node* DoSimplifyControlFlow::postorder(IR::EmptyStatement* statement)
const IR::Node* DoSimplifyControlFlow::postorder(IR::SwitchStatement* statement) {
LOG3("Visiting " << dbp(getOriginal()));
if (statement->cases.empty()) {
// The P4_16 spec prohibits expressions other than table application as
// switch conditions. The parser should have rejected programs for
// which this is not the case.
BUG_CHECK(statement->expression->is<IR::Member>(),
"%1%: expected a Member", statement->expression);
auto expr = statement->expression->to<IR::Member>();
BUG_CHECK(expr->expr->is<IR::MethodCallExpression>(),
"%1%: expected a table invocation", expr->expr);
auto mce = expr->expr->to<IR::MethodCallExpression>();
return new IR::MethodCallStatement(mce->srcInfo, mce);
// If this is a table application remove the switch altogether but keep
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Do we have a test for this?

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Also another test for a function call with side-effects could be useful.

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issue3619-1.p4 is testing the side-effect.

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But that function call is side-effect free. Also this does not include a table.apply(), right?

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This pass does not know that the function has no side effects. All tests with table applications would qualify for the second case.

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if (SideEffects::check(statement->expression, this, refMap, typeMap))
    // This can happen if this pass is run before SideEffectOrdering.
    return statement;

I am trying to understand this segment. If the function call has side effects, ideally only the method call statement should remain, right? I do not think we have a test for this.

Similarly,
if (TableApplySolver::isActionRun(statement->expression, refMap, typeMap)) {
should check whether there is an action_run call and then only leave the table apply statement.

I assume we have tests already for this case? Since this was the default behavior.

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@mihaibudiu mihaibudiu Nov 2, 2022 •
edited
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If the argument of the switch may have side-effects then we leave the whole switch unchaged. This pass is invoked again later, after SideEffectOrdering has removed calls with side effects from switch statements (all except table.apply().action_run), and that case will never occur again.

// the table application. Otherwise remove the switch altogether.
if (TableApplySolver::isActionRun(statement->expression, refMap, typeMap)) {
auto mce = statement->expression->checkedTo<IR::Member>()->
expr->checkedTo<IR::MethodCallExpression>();
LOG2("Removing switch statement " << statement << " keeping " << mce);
return new IR::MethodCallStatement(statement->srcInfo, mce);
}
if (SideEffects::check(statement->expression, this, refMap, typeMap))
// This can happen if this pass is run before SideEffectOrdering.
return statement;
LOG2("Removing switch statement " << statement);
return nullptr;
}
auto last = statement->cases.back();
if (last->statement == nullptr) {
Expand Down
15 changes: 15 additions & 0 deletions testdata/p4_16_samples/issue3619-1.p4
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@@ -0,0 +1,15 @@
bit<4> f(in bit<4> d) {
return d;
}

control c(in bit<4> v) {
apply {
switch(f(v)) {
}
}
}

control C(in bit<4> b);
package top(C c);

top(c()) main;
7 changes: 7 additions & 0 deletions testdata/p4_16_samples/issue3619.p4
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@@ -0,0 +1,7 @@
control c(in bit<4> v) {
apply {
switch(v) {
default:
}
}
}
13 changes: 13 additions & 0 deletions testdata/p4_16_samples_outputs/issue3619-1-first.p4
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@@ -0,0 +1,13 @@
bit<4> f(in bit<4> d) {
return d;
}
control c(in bit<4> v) {
apply {
switch (f(v)) {
}
}
}

control C(in bit<4> b);
package top(C c);
top(c()) main;
8 changes: 8 additions & 0 deletions testdata/p4_16_samples_outputs/issue3619-1-frontend.p4
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@@ -0,0 +1,8 @@
control c(in bit<4> v) {
apply {
}
}

control C(in bit<4> b);
package top(C c);
top(c()) main;
8 changes: 8 additions & 0 deletions testdata/p4_16_samples_outputs/issue3619-1-midend.p4
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@@ -0,0 +1,8 @@
control c(in bit<4> v) {
apply {
}
}

control C(in bit<4> b);
package top(C c);
top(c()) main;
13 changes: 13 additions & 0 deletions testdata/p4_16_samples_outputs/issue3619-1.p4
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@@ -0,0 +1,13 @@
bit<4> f(in bit<4> d) {
return d;
}
control c(in bit<4> v) {
apply {
switch (f(v)) {
}
}
}

control C(in bit<4> b);
package top(C c);
top(c()) main;
Empty file added testdata/p4_16_samples_outputs/issue3619-1.p4-stderr
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Empty file.
5 changes: 5 additions & 0 deletions testdata/p4_16_samples_outputs/issue3619-first.p4
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@@ -0,0 +1,5 @@
control c(in bit<4> v) {
apply {
}
}

Empty file added testdata/p4_16_samples_outputs/issue3619-frontend.p4
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Empty file.
8 changes: 8 additions & 0 deletions testdata/p4_16_samples_outputs/issue3619.p4
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@@ -0,0 +1,8 @@
control c(in bit<4> v) {
apply {
switch (v) {
default:
}
}
}

4 changes: 4 additions & 0 deletions testdata/p4_16_samples_outputs/issue3619.p4-stderr
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@@ -0,0 +1,4 @@
issue3619.p4(4): [--Wwarn=missing] warning: SwitchCase: fallthrough with no statement
default:
^^^^^^^
[--Wwarn=missing] warning: Program does not contain a `main' module