[llvm] [DropUnnecessaryAssumes] Make the ephemeral value check more precise (PR #160700)

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/160700

The initial implementation used a very crude check where a value was considered ephemeral if it has only one use. This is insufficient if there are multiple assumes acting on the same value, or in more complex cases like cyclic phis.

Generalize this to a more typical ephemeral value check, i.e. make sure that all transitive users are in assumes, while stopping at side-effecting instructions.

Interestingly, this doesn't seem to actually matter in practice (no difference on llvm-opt-benchmark).

>From 1e7289fff64d1aec0d5208f4451eed3e14abcece Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Thu, 25 Sep 2025 12:44:47 +0200
Subject: [PATCH] [DropUnnecessaryAssumes] Make the ephemeral value check more
 precise

The initial implementation used a very crude check where a value
was considered ephemeral if it has only one use. This is insufficient
if there are multiple assumes acting on the same value, or in more
complex cases like cyclic phis.

Generalize this to a more typical ephemeral value check, i.e. make
sure that all transitive users are in assumes, while stopping at
side-effecting instructions.
---
 .../Scalar/DropUnnecessaryAssumes.cpp         | 46 +++++++++--
 .../DropUnnecessaryAssumes/basic.ll           | 80 +++++++++++++++++++
 2 files changed, 119 insertions(+), 7 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp b/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp
index c215228b480d2..d4fb420211cb0 100644
--- a/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp
+++ b/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/Scalar/DropUnnecessaryAssumes.h"
+#include "llvm/ADT/SetVector.h"
 #include "llvm/Analysis/AssumptionCache.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/IntrinsicInst.h"
@@ -17,13 +18,44 @@ using namespace llvm;
 using namespace llvm::PatternMatch;
 
 static bool affectedValuesAreEphemeral(ArrayRef<Value *> Affected) {
-  // If all the affected uses have only one use (part of the assume), then
-  // the assume does not provide useful information. Note that additional
-  // users may appear as a result of inlining and CSE, so we should only
-  // make this assumption late in the optimization pipeline.
-  // TODO: Handle dead cyclic usages.
-  // TODO: Handle multiple dead assumes on the same value.
-  return all_of(Affected, match_fn(m_OneUse(m_Value())));
+  // Check whether all the uses are ephemeral, i.e. recursively only used
+  // by assumes. In that case, the assume does not provide useful information.
+  // Note that additional users may appear as a result of inlining and CSE,
+  // so we should only make this assumption late in the optimization pipeline.
+  SmallSetVector<User *, 16> Worklist;
+  auto AddUser = [&](User *U) {
+    // Bail out if we need to inspect too many users.
+    if (Worklist.size() >= 32)
+      return false;
+    Worklist.insert(U);
+    return true;
+  };
+
+  for (Value *V : Affected)
+    for (User *U : V->users())
+      if (!AddUser(U))
+        return false;
+
+  for (unsigned Idx = 0; Idx < Worklist.size(); ++Idx) {
+    auto *I = dyn_cast<Instruction>(Worklist[Idx]);
+    if (!I)
+      return false;
+
+    // Use in assume is ephemeral.
+    if (isa<AssumeInst>(I))
+      continue;
+
+    // Use in side-effecting instruction is non-ephemeral.
+    if (I->mayHaveSideEffects() || I->isTerminator())
+      return false;
+
+    // Otherwise, recursively look at the users.
+    for (User *NestedU : I->users())
+      if (!AddUser(NestedU))
+        return false;
+  }
+
+  return true;
 }
 
 PreservedAnalyses
diff --git a/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll b/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll
index e2a9b4eea2c7d..a61dc2dcc4b9f 100644
--- a/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll
+++ b/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll
@@ -1,6 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
 ; RUN: opt -S -passes=drop-unnecessary-assumes < %s | FileCheck %s
 
+declare void @dummy(i32 %x)
+
 define void @basic_dead(i32 %x) {
 ; CHECK-LABEL: define void @basic_dead(
 ; CHECK-SAME: i32 [[X:%.*]]) {
@@ -180,3 +182,81 @@ define void @type_test(ptr %x) {
   call void @llvm.assume(i1 %test)
   ret void
 }
+
+define void @multiple_dead_conds(i32 %x) {
+; CHECK-LABEL: define void @multiple_dead_conds(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:    ret void
+;
+  %cond1 = icmp sge i32 %x, 0
+  call void @llvm.assume(i1 %cond1)
+  %cond2 = icmp ne i32 %x, 64
+  call void @llvm.assume(i1 %cond2)
+  ret void
+}
+
+define void @multiple_dead_bundles(ptr %x) {
+; CHECK-LABEL: define void @multiple_dead_bundles(
+; CHECK-SAME: ptr [[X:%.*]]) {
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.assume(i1 true) ["align"(ptr %x, i64 8), "nonnull"(ptr %x)]
+  ret void
+}
+
+; The assume is eliminated, but currently leaves behind a dead cycle.
+define void @dead_cycle(i1 %loop.cond) {
+; CHECK-LABEL: define void @dead_cycle(
+; CHECK-SAME: i1 [[LOOP_COND:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    br label %[[LOOP:.*]]
+; CHECK:       [[LOOP]]:
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[IV_NEXT]] = add i32 [[IV]], 1
+; CHECK-NEXT:    br i1 [[LOOP_COND]], label %[[LOOP]], label %[[EXIT:.*]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop ]
+  %cond = icmp ne i32 %iv, 64
+  call void @llvm.assume(i1 %cond)
+  %iv.next = add i32 %iv, 1
+  br i1 %loop.cond, label %loop, label %exit
+
+exit:
+  ret void
+}
+
+define void @use_in_side_effect(i32 %x) {
+; CHECK-LABEL: define void @use_in_side_effect(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:    [[COND:%.*]] = icmp sge i32 [[X]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[COND]])
+; CHECK-NEXT:    call void @dummy(i32 [[X]])
+; CHECK-NEXT:    ret void
+;
+  %cond = icmp sge i32 %x, 0
+  call void @llvm.assume(i1 %cond)
+  call void @dummy(i32 %x)
+  ret void
+}
+
+define void @indirect_use_in_side_effect(i32 %x) {
+; CHECK-LABEL: define void @indirect_use_in_side_effect(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:    [[COND:%.*]] = icmp sge i32 [[X]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[COND]])
+; CHECK-NEXT:    [[ADD:%.*]] = add i32 [[X]], 1
+; CHECK-NEXT:    call void @dummy(i32 [[ADD]])
+; CHECK-NEXT:    ret void
+;
+  %cond = icmp sge i32 %x, 0
+  call void @llvm.assume(i1 %cond)
+  %add = add i32 %x, 1
+  call void @dummy(i32 %add)
+  ret void
+}