[Clang] Fix crash in coverage of if consteval.

Clang crashes when encountering an `if consteval` statement.
This is the minimum fix not to crash.
The fix is consistent with the current behavior of if constexpr,
which does generate coverage data for the discarded branches.
This is of course not correct and a better solution is
needed for both if constexpr and if consteval.
See llvm/llvm-project#54419.

Fixes #57377

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D132723

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -206,6 +206,9 @@ Bug Fixes
   missing when used, or vice versa. This makes sure that Clang picks the
   correct one, where it previously would consider multiple ones as potentially
   acceptable (and erroneously use whichever one is tried first).
+- Fix a crash when generating code coverage information for an
+  ``if consteval`` statement. This fixes
+  `Issue 57377 <https://github.com/llvm/llvm-project/issues/57377>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/CodeGen/CoverageMappingGen.cpp

@@ -1377,19 +1377,23 @@ struct CounterCoverageMappingBuilder
 
     // Extend into the condition before we propagate through it below - this is
     // needed to handle macros that generate the "if" but not the condition.
-    extendRegion(S->getCond());
+    if (!S->isConsteval())
+      extendRegion(S->getCond());
 
     Counter ParentCount = getRegion().getCounter();
     Counter ThenCount = getRegionCounter(S);
 
-    // Emitting a counter for the condition makes it easier to interpret the
-    // counter for the body when looking at the coverage.
-    propagateCounts(ParentCount, S->getCond());
+    if (!S->isConsteval()) {
+      // Emitting a counter for the condition makes it easier to interpret the
+      // counter for the body when looking at the coverage.
+      propagateCounts(ParentCount, S->getCond());
 
-    // The 'then' count applies to the area immediately after the condition.
-    auto Gap = findGapAreaBetween(S->getRParenLoc(), getStart(S->getThen()));
-    if (Gap)
-      fillGapAreaWithCount(Gap->getBegin(), Gap->getEnd(), ThenCount);
+      // The 'then' count applies to the area immediately after the condition.
+      Optional<SourceRange> Gap =
+          findGapAreaBetween(S->getRParenLoc(), getStart(S->getThen()));
+      if (Gap)
+        fillGapAreaWithCount(Gap->getBegin(), Gap->getEnd(), ThenCount);
+    }
 
     extendRegion(S->getThen());
     Counter OutCount = propagateCounts(ThenCount, S->getThen());
@@ -1398,9 +1402,9 @@ struct CounterCoverageMappingBuilder
     if (const Stmt *Else = S->getElse()) {
       bool ThenHasTerminateStmt = HasTerminateStmt;
       HasTerminateStmt = false;
-
       // The 'else' count applies to the area immediately after the 'then'.
-      Gap = findGapAreaBetween(getEnd(S->getThen()), getStart(Else));
+      Optional<SourceRange> Gap =
+          findGapAreaBetween(getEnd(S->getThen()), getStart(Else));
       if (Gap)
         fillGapAreaWithCount(Gap->getBegin(), Gap->getEnd(), ElseCount);
       extendRegion(Else);
@@ -1416,9 +1420,11 @@ struct CounterCoverageMappingBuilder
       GapRegionCounter = OutCount;
     }
 
-    // Create Branch Region around condition.
-    createBranchRegion(S->getCond(), ThenCount,
-                       subtractCounters(ParentCount, ThenCount));
+    if (!S->isConsteval()) {
+      // Create Branch Region around condition.
+      createBranchRegion(S->getCond(), ThenCount,
+                         subtractCounters(ParentCount, ThenCount));
+    }
   }
 
   void VisitCXXTryStmt(const CXXTryStmt *S) {

clang/test/CoverageMapping/if.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -mllvm -emptyline-comment-coverage=false -fprofile-instrument=clang -fcoverage-mapping -dump-coverage-mapping -emit-llvm-only -std=c++1z -triple %itanium_abi_triple -main-file-name if.cpp %s | FileCheck %s
+// RUN: %clang_cc1 -mllvm -emptyline-comment-coverage=false -fprofile-instrument=clang -fcoverage-mapping -dump-coverage-mapping -emit-llvm-only -std=c++2b -triple %itanium_abi_triple -main-file-name if.cpp %s | FileCheck %s
 
 int nop() { return 0; }
 
@@ -13,6 +13,22 @@ void foo() {                    // CHECK-NEXT: Gap,File 0, [[@LINE+1]]:21 -> [[@
     ++j;                        // CHECK-NEXT: Branch,File 0, [[@LINE-1]]:7 -> [[@LINE-1]]:8 = #1, (#0 - #1)
 }                               // CHECK-NEXT: [[@LINE-2]]:9 -> [[@LINE-1]]:5 = #1
                                 // CHECK-NEXT: [[@LINE-2]]:5 -> [[@LINE-2]]:8 = #1
+
+// FIXME: Do not generate coverage for discarded branches in if consteval and if constexpr statements
+constexpr int check_consteval(int i) {
+    if consteval {
+      i++;
+    }
+    if !consteval {
+      i++;
+    }
+    if consteval {
+        return 42;
+    } else {
+        return i;
+    }
+}
+
 // CHECK-LABEL: main:
 int main() {                    // CHECK: File 0, [[@LINE]]:12 -> {{[0-9]+}}:2 = #0
   int i = 0;
@@ -50,6 +66,10 @@ int main() {                    // CHECK: File 0, [[@LINE]]:12 -> {{[0-9]+}}:2 =
                                 // CHECK-NEXT: File 0, [[@LINE+1]]:14 -> [[@LINE+1]]:20 = #6
   i = i == 0?i + 12:i + 10;     // CHECK-NEXT: File 0, [[@LINE]]:21 -> [[@LINE]]:27 = (#0 - #6)
 
+  // GH-57377
+  constexpr int c_i = check_consteval(0);
+  check_consteval(i);
+
   return 0;
 }