Attributor: Propagate align to cmpxchg instructions

arsenm · arsenm · commit 4d6890e6ca69 · 2025-04-08T15:12:31.000Z
Fixes #134480
diff --git a/llvm/lib/Transforms/IPO/AttributorAttributes.cpp b/llvm/lib/Transforms/IPO/AttributorAttributes.cpp
@@ -5317,6 +5317,15 @@ struct AAAlignImpl : AAAlign {
             InstrChanged = ChangeStatus::CHANGED;
           }
         }
+      } else if (auto *CAS = dyn_cast<AtomicCmpXchgInst>(U.getUser())) {
+        if (CAS->getPointerOperand() == &AssociatedValue) {
+          if (CAS->getAlign() < getAssumedAlign()) {
+            STATS_DECLTRACK(AAAlign, AtomicCmpXchg,
+                            "Number of times alignment added to cmpxchg");
+            CAS->setAlignment(getAssumedAlign());
+            InstrChanged = ChangeStatus::CHANGED;
+          }
+        }
       }
     }
 
diff --git a/llvm/test/Transforms/Attributor/align-atomic.ll b/llvm/test/Transforms/Attributor/align-atomic.ll
@@ -37,7 +37,7 @@ define ptr @atomicrmw_non_ptr_op_no_propagate(ptr %ptr, ptr align 16 %val) {
 define i32 @cmpxchg_propagate(ptr align 8 %ptr, i32 %cmp, i32 %val) {
 ; CHECK-LABEL: define i32 @cmpxchg_propagate(
 ; CHECK-SAME: ptr nofree noundef nonnull align 8 captures(none) dereferenceable(4) [[PTR:%.*]], i32 [[CMP:%.*]], i32 [[VAL:%.*]]) #[[ATTR0]] {
-; CHECK-NEXT:    [[PAIR:%.*]] = cmpxchg ptr [[PTR]], i32 [[CMP]], i32 [[VAL]] seq_cst monotonic, align 2
+; CHECK-NEXT:    [[PAIR:%.*]] = cmpxchg ptr [[PTR]], i32 [[CMP]], i32 [[VAL]] seq_cst monotonic, align 8
 ; CHECK-NEXT:    [[RESULT:%.*]] = extractvalue { i32, i1 } [[PAIR]], 0
 ; CHECK-NEXT:    ret i32 [[RESULT]]
 ;

Original file line number	Diff line number	Diff line change
`@@ -5317,6 +5317,15 @@ struct AAAlignImpl : AAAlign {`
`5317`	`5317`	`InstrChanged = ChangeStatus::CHANGED;`
`5318`	`5318`	`}`
`5319`	`5319`	`}`
	`5320`	`+ } else if (auto *CAS = dyn_cast<AtomicCmpXchgInst>(U.getUser())) {`
	`5321`	`+ if (CAS->getPointerOperand() == &AssociatedValue) {`
	`5322`	`+ if (CAS->getAlign() < getAssumedAlign()) {`
	`5323`	`+ STATS_DECLTRACK(AAAlign, AtomicCmpXchg,`
	`5324`	`+ "Number of times alignment added to cmpxchg");`
	`5325`	`+ CAS->setAlignment(getAssumedAlign());`
	`5326`	`+ InstrChanged = ChangeStatus::CHANGED;`
	`5327`	`+ }`
	`5328`	`+ }`
`5320`	`5329`	`}`
`5321`	`5330`	`}`
`5322`	`5331`
Original file line number	Diff line number	Diff line change
`@@ -37,7 +37,7 @@ define ptr @atomicrmw_non_ptr_op_no_propagate(ptr %ptr, ptr align 16 %val) {`
`37`	`37`	`define i32 @cmpxchg_propagate(ptr align 8 %ptr, i32 %cmp, i32 %val) {`
`38`	`38`	`; CHECK-LABEL: define i32 @cmpxchg_propagate(`
`39`	`39`	`; CHECK-SAME: ptr nofree noundef nonnull align 8 captures(none) dereferenceable(4) [[PTR:%.]], i32 [[CMP:%.]], i32 [[VAL:%.*]]) #[[ATTR0]] {`
`40`		`-; CHECK-NEXT: [[PAIR:%.*]] = cmpxchg ptr [[PTR]], i32 [[CMP]], i32 [[VAL]] seq_cst monotonic, align 2`
	`40`	`+; CHECK-NEXT: [[PAIR:%.*]] = cmpxchg ptr [[PTR]], i32 [[CMP]], i32 [[VAL]] seq_cst monotonic, align 8`
`41`	`41`	`; CHECK-NEXT: [[RESULT:%.*]] = extractvalue { i32, i1 } [[PAIR]], 0`
`42`	`42`	`; CHECK-NEXT: ret i32 [[RESULT]]`
`43`	`43`	`;`