[llvm] [FunctionAttrs] Treat byval calls as only reading ptrs (PR #122618)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Alex MacLean (AlexMaclean)

<details>
<summary>Changes</summary>

Since byval arguments are passed via a hidden copy of the pointee, they do not have the same semantics as normal pointer arguments. The callee cannot capture or write to the pointer and the copy is a read of the pointer. 

---
Full diff: https://github.com/llvm/llvm-project/pull/122618.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/IPO/FunctionAttrs.cpp (+4-3) 
- (modified) llvm/test/Transforms/FunctionAttrs/readattrs.ll (+22) 


``````````diff
diff --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
index fe9cca01a8f31f..7ac9b70f47902c 100644
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -852,9 +852,10 @@ determinePointerAccessAttrs(Argument *A,
         continue;
       }
 
-      // Given we've explictily handled the callee operand above, what's left
+      // Given we've explicitly handled the callee operand above, what's left
       // must be a data operand (e.g. argument or operand bundle)
       const unsigned UseIndex = CB.getDataOperandNo(U);
+      const bool IsByVal = CB.isByValArgument(CB.getArgOperandNo(U));
 
       // Some intrinsics (for instance ptrmask) do not capture their results,
       // but return results thas alias their pointer argument, and thus should
@@ -864,7 +865,7 @@ determinePointerAccessAttrs(Argument *A,
         for (Use &UU : CB.uses())
           if (Visited.insert(&UU).second)
             Worklist.push_back(&UU);
-      } else if (!CB.doesNotCapture(UseIndex)) {
+      } else if (!(CB.doesNotCapture(UseIndex) || IsByVal)) {
         if (!CB.onlyReadsMemory())
           // If the callee can save a copy into other memory, then simply
           // scanning uses of the call is insufficient.  We have no way
@@ -894,7 +895,7 @@ determinePointerAccessAttrs(Argument *A,
       // invokes with operand bundles.
       if (CB.doesNotAccessMemory(UseIndex)) {
         /* nop */
-      } else if (!isModSet(ArgMR) || CB.onlyReadsMemory(UseIndex)) {
+      } else if (!isModSet(ArgMR) || CB.onlyReadsMemory(UseIndex) || IsByVal) {
         IsRead = true;
       } else if (!isRefSet(ArgMR) ||
                  CB.dataOperandHasImpliedAttr(UseIndex, Attribute::WriteOnly)) {
diff --git a/llvm/test/Transforms/FunctionAttrs/readattrs.ll b/llvm/test/Transforms/FunctionAttrs/readattrs.ll
index 004c0485d764ae..6087160d13ca1d 100644
--- a/llvm/test/Transforms/FunctionAttrs/readattrs.ll
+++ b/llvm/test/Transforms/FunctionAttrs/readattrs.ll
@@ -762,5 +762,27 @@ define void @writable_readnone(ptr writable dereferenceable(4) %p) {
   ret void
 }
 
+declare void @byval_param(ptr byval(i32) %p)
+
+define void @call_byval_param(ptr %p) {
+; FNATTRS-LABEL: define {{[^@]+}}@call_byval_param
+; FNATTRS-SAME: (ptr readonly [[P:%.*]]) {
+; FNATTRS-NEXT:    call void @byval_param(ptr byval(i32) [[P]])
+; FNATTRS-NEXT:    ret void
+;
+; ATTRIBUTOR-LABEL: define {{[^@]+}}@call_byval_param
+; ATTRIBUTOR-SAME: (ptr nocapture readonly [[P:%.*]]) {
+; ATTRIBUTOR-NEXT:    call void @byval_param(ptr nocapture readonly byval(i32) [[P]])
+; ATTRIBUTOR-NEXT:    ret void
+;
+; ATTRIBUTOR-CGSCC-LABEL: define {{[^@]+}}@call_byval_param
+; ATTRIBUTOR-CGSCC-SAME: (ptr nocapture readonly [[P:%.*]]) {
+; ATTRIBUTOR-CGSCC-NEXT:    call void @byval_param(ptr nocapture readonly byval(i32) [[P]])
+; ATTRIBUTOR-CGSCC-NEXT:    ret void
+;
+  call void @byval_param(ptr byval(i32) %p)
+  ret void
+}
+
 ;; NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
 ; COMMON: {{.*}}

``````````

</details>


https://github.com/llvm/llvm-project/pull/122618