Skip to content

[OpenMP] Conditional modifier on lastprivate clause is producing incorrect result in C mode #156004

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom
Merged
merged 3 commits into from
Sep 11, 2025
Merged

[OpenMP] Conditional modifier on lastprivate clause is producing incorrect result in C mode #156004

merged 3 commits into from
Sep 11, 2025

Conversation

chandraghale
Copy link
Contributor

@chandraghale chandraghale commented Aug 29, 2025 •
edited
Loading

Conditional modifier on lastprivate clause is producing incorrect result when compiled with clang( C compiler). IR is not emitting while compilation with C compiler.
However it is working correctly with clang++
The OpenMP hook that emits the conditional modifier (checkAndEmitLastprivateConditional) is skipped in C because assignment is a prvalue and takes the scalar path.
Original Codegen Support : eddb8

C  = → prvalue → EmitAnyExpr(TEK_Scalar) → ScalarExprEmitter::VisitBinAssign (hook not reached)
C++ = → lvalue → EmitBinaryOperatorLValue

Failing Test Case :
#include <stdio.h>
#define N 10

int A[N];
void condlastprivate() {
int x, y, z, k;
x = y = z = k = 0;
#pragma omp parallel for lastprivate(conditional: x,y, z) lastprivate(k)
for( k=0; k<N; k++){
if ((k >2 ) && (k <6))
{ x = A[k]; z = A[k]+111; }
else
{ y = A[k]+222; }
}
printf("Expecting: x=5, y=231, z=116 k=10 Got: x=%d y=%d z=%d k=%d \n", x,y,z,k);
}
int main() {
for( int i=0; i<N; i++)
{ A[i] = i; }
condlastprivate();
return 0;
}

#>./clang  -fopenmp  cond_c.c 
#> ./a.out 
Expecting: x=5, y=231, z=116 k=10 **Got: x=-1376379760 y=231 z=631465600** k=10

@llvmbot llvmbot added clang Clang issues not falling into any other category clang:codegen IR generation bugs: mangling, exceptions, etc. clang:openmp OpenMP related changes to Clang labels Aug 29, 2025
@llvmbot
Copy link
Member

llvmbot commented Aug 29, 2025

@llvm/pr-subscribers-clang

Author: CHANDRA GHALE (chandraghale)

Changes

Conditional modifier on lastprivate clause is producing incorrect result when compiled with clang( C compiler). IR is not emitting while compilation with C compiler.
However it is working correctly with clang++
The OpenMP hook that emits the conditional modifier (checkAndEmitLastprivateConditional) is skipped in C because assignment is a prvalue and takes the scalar path.

C  = → prvalue → EmitAnyExpr(TEK_Scalar) → ScalarExprEmitter::VisitBinAssign (hook not reached)
C++ = → lvalue → EmitBinaryOperatorLValue

Failing Test Case :
#include &lt;stdio.h&gt;
#define N 10

int A[N];
void condlastprivate() {
int x, y, z, k;
x = y = z = k = 0;
#pragma omp parallel for lastprivate(conditional: x,y, z) lastprivate(k)
for( k=0; k&lt;N; k++){
if ((k &gt;2 ) &amp;&amp; (k &lt;6))
{ x = A[k]; z = A[k]+111; }
else
{ y = A[k]+222; }
}
printf("Expecting: x=5, y=231, z=116 k=10 Got: x=%d y=%d z=%d k=%d \n", x,y,z,k);
}
int main() {
for( int i=0; i&lt;N; i++)
{ A[i] = i; }
condlastprivate();
return 0;
}

#&gt;./clang  -fopenmp  cond_c.c 
#&gt; ./a.out 
Expecting: x=5, y=231, z=116 k=10 **Got: x=-1376379760 y=231 z=631465600** k=10

Full diff: https://github.com/llvm/llvm-project/pull/156004.diff

2 Files Affected:

  • (modified) clang/lib/CodeGen/CGExprScalar.cpp (+5)
  • (added) clang/test/OpenMP/for_lst_private_codegen_c.c (+53)
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 193710bef2d16..d37712f27ed5e 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -5183,6 +5183,11 @@ Value *ScalarExprEmitter::VisitBinAssign(const BinaryOperator *E) {
       CGF.EmitStoreThroughLValue(RValue::get(RHS), LHS);
     }
   }
+  // OpenMP: Handle lastprivate(condition:) in scalar assignment
+  if (CGF.getLangOpts().OpenMP) {
+      CGF.CGM.getOpenMPRuntime().checkAndEmitLastprivateConditional(CGF,
+                                                                    E->getLHS());
+  }
 
   // If the result is clearly ignored, return now.
   if (Ignore)
diff --git a/clang/test/OpenMP/for_lst_private_codegen_c.c b/clang/test/OpenMP/for_lst_private_codegen_c.c
new file mode 100644
index 0000000000000..c56c5b6fb9351
--- /dev/null
+++ b/clang/test/OpenMP/for_lst_private_codegen_c.c
@@ -0,0 +1,53 @@
+// RUN: %clang_cc1 -verify -x c -triple x86_64-unknown-linux-gnu -fopenmp -fopenmp-version=52  -emit-llvm -o - %s | FileCheck %s
+// expected-no-diagnostics
+
+#define N 100
+int A[N];
+
+void condlastprivate() {
+  int x, y, z, k;
+  x = y = z = k = 0;
+
+  #pragma omp parallel for lastprivate(conditional: x,y,z) lastprivate(k)
+  for (k = 0; k < N; k++) {
+    if ((k > 2) && (k < 6)) {
+      x = A[k];
+      z = A[k] + 111;
+    } else {
+      y = A[k] + 222;
+    }
+  }
+}
+
+int main() {
+  for (int i = 0; i < N; i++)
+    A[i] = i;
+  condlastprivate();
+  return 0;
+}
+
+// CHECK: @.pl_cond.x_[[ID:[0-9]+]].iv = common global i32 0, align 4
+// CHECK: @pl_cond.x_[[ID]] = common global i32 0, align 4
+// CHECK: @.gomp_critical_user_pl_cond.x_[[ID]].var = common global [8 x i32] zeroinitializer, align 8
+
+// CHECK: @.pl_cond.z_[[ID]].iv = common global i32 0, align 4
+// CHECK: @pl_cond.z_[[ID]] = common global i32 0, align 4
+// CHECK: @.gomp_critical_user_pl_cond.z_[[ID]].var = common global [8 x i32] zeroinitializer, align 8
+
+// CHECK: @.pl_cond.y_[[ID]].iv = common global i32 0, align 4
+// CHECK: @pl_cond.y_[[ID]] = common global i32 0, align 4
+// CHECK: @.gomp_critical_user_pl_cond.y_[[ID]].var = common global [8 x i32] zeroinitializer, align 8
+
+// CHECK-LABEL: define internal void @condlastprivate.omp_outlined(
+// CHECK: call void @__kmpc_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.x_[[ID]].var)
+// CHECK: store i32 %{{[0-9]+}}, ptr @pl_cond.x_[[ID]], align 4
+// CHECK: call void @__kmpc_end_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.x_[[ID]].var)
+
+// CHECK: call void @__kmpc_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.z_[[ID]].var)
+// CHECK: store i32 %{{[0-9]+}}, ptr @pl_cond.z_[[ID]], align 4
+// CHECK: call void @__kmpc_end_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.z_[[ID]].var)
+
+// CHECK: call void @__kmpc_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.y_[[ID]].var)
+// CHECK: store i32 %{{[0-9]+}}, ptr @pl_cond.y_[[ID]], align 4
+// CHECK: call void @__kmpc_end_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.y_[[ID]].var)
+

@llvmbot
Copy link
Member

llvmbot commented Aug 29, 2025

@llvm/pr-subscribers-clang-codegen

Author: CHANDRA GHALE (chandraghale)

Changes

Conditional modifier on lastprivate clause is producing incorrect result when compiled with clang( C compiler). IR is not emitting while compilation with C compiler.
However it is working correctly with clang++
The OpenMP hook that emits the conditional modifier (checkAndEmitLastprivateConditional) is skipped in C because assignment is a prvalue and takes the scalar path.

C  = → prvalue → EmitAnyExpr(TEK_Scalar) → ScalarExprEmitter::VisitBinAssign (hook not reached)
C++ = → lvalue → EmitBinaryOperatorLValue

Failing Test Case :
#include &lt;stdio.h&gt;
#define N 10

int A[N];
void condlastprivate() {
int x, y, z, k;
x = y = z = k = 0;
#pragma omp parallel for lastprivate(conditional: x,y, z) lastprivate(k)
for( k=0; k&lt;N; k++){
if ((k &gt;2 ) &amp;&amp; (k &lt;6))
{ x = A[k]; z = A[k]+111; }
else
{ y = A[k]+222; }
}
printf("Expecting: x=5, y=231, z=116 k=10 Got: x=%d y=%d z=%d k=%d \n", x,y,z,k);
}
int main() {
for( int i=0; i&lt;N; i++)
{ A[i] = i; }
condlastprivate();
return 0;
}

#&gt;./clang  -fopenmp  cond_c.c 
#&gt; ./a.out 
Expecting: x=5, y=231, z=116 k=10 **Got: x=-1376379760 y=231 z=631465600** k=10

Full diff: https://github.com/llvm/llvm-project/pull/156004.diff

2 Files Affected:

  • (modified) clang/lib/CodeGen/CGExprScalar.cpp (+5)
  • (added) clang/test/OpenMP/for_lst_private_codegen_c.c (+53)
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 193710bef2d16..d37712f27ed5e 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -5183,6 +5183,11 @@ Value *ScalarExprEmitter::VisitBinAssign(const BinaryOperator *E) {
       CGF.EmitStoreThroughLValue(RValue::get(RHS), LHS);
     }
   }
+  // OpenMP: Handle lastprivate(condition:) in scalar assignment
+  if (CGF.getLangOpts().OpenMP) {
+      CGF.CGM.getOpenMPRuntime().checkAndEmitLastprivateConditional(CGF,
+                                                                    E->getLHS());
+  }
 
   // If the result is clearly ignored, return now.
   if (Ignore)
diff --git a/clang/test/OpenMP/for_lst_private_codegen_c.c b/clang/test/OpenMP/for_lst_private_codegen_c.c
new file mode 100644
index 0000000000000..c56c5b6fb9351
--- /dev/null
+++ b/clang/test/OpenMP/for_lst_private_codegen_c.c
@@ -0,0 +1,53 @@
+// RUN: %clang_cc1 -verify -x c -triple x86_64-unknown-linux-gnu -fopenmp -fopenmp-version=52  -emit-llvm -o - %s | FileCheck %s
+// expected-no-diagnostics
+
+#define N 100
+int A[N];
+
+void condlastprivate() {
+  int x, y, z, k;
+  x = y = z = k = 0;
+
+  #pragma omp parallel for lastprivate(conditional: x,y,z) lastprivate(k)
+  for (k = 0; k < N; k++) {
+    if ((k > 2) && (k < 6)) {
+      x = A[k];
+      z = A[k] + 111;
+    } else {
+      y = A[k] + 222;
+    }
+  }
+}
+
+int main() {
+  for (int i = 0; i < N; i++)
+    A[i] = i;
+  condlastprivate();
+  return 0;
+}
+
+// CHECK: @.pl_cond.x_[[ID:[0-9]+]].iv = common global i32 0, align 4
+// CHECK: @pl_cond.x_[[ID]] = common global i32 0, align 4
+// CHECK: @.gomp_critical_user_pl_cond.x_[[ID]].var = common global [8 x i32] zeroinitializer, align 8
+
+// CHECK: @.pl_cond.z_[[ID]].iv = common global i32 0, align 4
+// CHECK: @pl_cond.z_[[ID]] = common global i32 0, align 4
+// CHECK: @.gomp_critical_user_pl_cond.z_[[ID]].var = common global [8 x i32] zeroinitializer, align 8
+
+// CHECK: @.pl_cond.y_[[ID]].iv = common global i32 0, align 4
+// CHECK: @pl_cond.y_[[ID]] = common global i32 0, align 4
+// CHECK: @.gomp_critical_user_pl_cond.y_[[ID]].var = common global [8 x i32] zeroinitializer, align 8
+
+// CHECK-LABEL: define internal void @condlastprivate.omp_outlined(
+// CHECK: call void @__kmpc_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.x_[[ID]].var)
+// CHECK: store i32 %{{[0-9]+}}, ptr @pl_cond.x_[[ID]], align 4
+// CHECK: call void @__kmpc_end_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.x_[[ID]].var)
+
+// CHECK: call void @__kmpc_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.z_[[ID]].var)
+// CHECK: store i32 %{{[0-9]+}}, ptr @pl_cond.z_[[ID]], align 4
+// CHECK: call void @__kmpc_end_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.z_[[ID]].var)
+
+// CHECK: call void @__kmpc_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.y_[[ID]].var)
+// CHECK: store i32 %{{[0-9]+}}, ptr @pl_cond.y_[[ID]], align 4
+// CHECK: call void @__kmpc_end_critical(ptr @2, {{.*}}, ptr @.gomp_critical_user_pl_cond.y_[[ID]].var)
+

@github-actions GitHub Actions
Copy link

github-actions bot commented Aug 29, 2025 •
edited
Loading

✅ With the latest revision this PR passed the C/C++ code formatter.

Chandra Ghale and others added 2 commits August 29, 2025 06:03
@chandraghale chandraghale requested review from alexey-bataev and removed request for alexey-bataev September 8, 2025 15:55
@chandraghale chandraghale merged commit 2f755c5 into llvm:main Sep 11, 2025
9 checks passed
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Reviewers

@alexey-bataev alexey-bataev alexey-bataev approved these changes

@shiltian shiltian Awaiting requested review from shiltian

Assignees
No one assigned
Labels
clang:codegen IR generation bugs: mangling, exceptions, etc. clang:openmp OpenMP related changes to Clang clang Clang issues not falling into any other category
Projects
None yet
Milestone
No milestone
Development

Successfully merging this pull request may close these issues.
3 participants
@chandraghale @llvmbot @alexey-bataev