[clang] [clang] Incorrect IR involving the use of bcopy (PR #79298)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-codegen

Author: None (vojkan99)

<details>
<summary>Changes</summary>

This patch addresses the issue regarding the call of bcopy function in a 
conditional expression.
It is analogous to the already accepted patch which deals with the same 
problem, just regarding the bzero function [0].

Here is the testcase which illustrates the issue:

```
void bcopy(const void *, void *, unsigned long);
void foo(void);

void test_bcopy() {
  char dst[20];
  char src[20];
  int _sz = 20, len = 20;
  return (_sz
          ? ((_sz &gt;= len)
             ? bcopy(src, dst, len)
             : foo())
          : bcopy(src, dst, len));
}
```

When processing it with clang, following issue occurs:

Instruction does not dominate all uses!
  %arraydecay2 = getelementptr inbounds [20 x i8], ptr %dst, i64 0, i64 0, !dbg 
!38
  %cond = phi ptr [ %arraydecay2, %cond.end ], [ %arraydecay5, %cond.false3 ], 
!dbg !33
fatal error: error in backend: Broken module found, compilation aborted!

This happens because an incorrect phi node is created. It is created because 
bcopy function call is lowered to the call of llvm.memmove intrinsic and 
function memmove returns void *. Since llvm.memmove is called in two places in 
the same return statement, clang creates a phi node in the final basic block 
for the return value and that phi node is incorrect. However, bcopy function 
should return void in the first place, so this phi node is unnecessary. This is 
what this patch addresses. An appropriate test is also added and no existing 
tests fail when applying this patch.

Also, this crash only happens when LLVM is configured with 
-DLLVM_ENABLE_ASSERTIONS=On option.

[0] https://reviews.llvm.org/D39746

---
Full diff: https://github.com/llvm/llvm-project/pull/79298.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGBuiltin.cpp (+1-1) 
- (modified) clang/test/CodeGen/builtins.c (+15) 


``````````diff
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 7ef764b8e1ac80e..bfe2e45d545322f 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -4011,7 +4011,7 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl 
GD, unsigned BuiltinID,
     EmitNonNullArgCheck(RValue::get(Dest.getPointer()), 
E->getArg(1)->getType(),
                         E->getArg(1)->getExprLoc(), FD, 0);
     Builder.CreateMemMove(Dest, Src, SizeVal, false);
-    return RValue::get(Dest.getPointer());
+    return RValue::get(nullptr);
   }
 
   case Builtin::BImemcpy:
diff --git a/clang/test/CodeGen/builtins.c b/clang/test/CodeGen/builtins.c
index ce1182b724dcc21..ed03233b6f1a967 100644
--- a/clang/test/CodeGen/builtins.c
+++ b/clang/test/CodeGen/builtins.c
@@ -202,6 +202,21 @@ void test_conditional_bzero(void) {
   // CHECK-NOT: phi
 }
 
+// CHECK-LABEL: define{{.*}} void @test_conditional_bcopy
+void test_conditional_bcopy(void) {
+  char dst[20];
+  char src[20];
+  int _sz = 20, len = 20;
+  return (_sz
+          ? ((_sz >= len)
+              ? __builtin_bcopy(src, dst, len)
+              : foo())
+          : __builtin_bcopy(src, dst, len));
+  // CHECK: call void @llvm.memmove
+  // CHECK: call void @llvm.memmove
+  // CHECK-NOT: phi
+}
+
 // CHECK-LABEL: define{{.*}} void @test_float_builtins
 void test_float_builtins(__fp16 *H, float F, double D, long double LD) {
   volatile int res;

``````````

</details>


https://github.com/llvm/llvm-project/pull/79298
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