From 1e7289fff64d1aec0d5208f4451eed3e14abcece Mon Sep 17 00:00:00 2001 From: Nikita Popov Date: Thu, 25 Sep 2025 12:44:47 +0200 Subject: [PATCH 1/5] [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 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 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(Worklist[Idx]); + if (!I) + return false; + + // Use in assume is ephemeral. + if (isa(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 +} From b390aa48d89d83c47bf27961021e27ce948983e4 Mon Sep 17 00:00:00 2001 From: Nikita Popov Date: Fri, 26 Sep 2025 11:55:40 +0200 Subject: [PATCH 2/5] Add test for affected value having side effect --- .../DropUnnecessaryAssumes/basic.ll | 37 ++++++++++++++++--- 1 file changed, 32 insertions(+), 5 deletions(-) diff --git a/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll b/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll index a61dc2dcc4b9f..ad1912e0c4662 100644 --- a/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll +++ b/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll @@ -1,7 +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) +declare void @use(i32 %x) +declare i32 @get() define void @basic_dead(i32 %x) { ; CHECK-LABEL: define void @basic_dead( @@ -236,12 +237,12 @@ define void @use_in_side_effect(i32 %x) { ; 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: call void @use(i32 [[X]]) ; CHECK-NEXT: ret void ; %cond = icmp sge i32 %x, 0 call void @llvm.assume(i1 %cond) - call void @dummy(i32 %x) + call void @use(i32 %x) ret void } @@ -251,12 +252,38 @@ define void @indirect_use_in_side_effect(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: call void @use(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) + call void @use(i32 %add) ret void } + +; The affected value itself has a side effect, but we can still drop the +; assume. +define void @affected_value_has_side_effect() { +; CHECK-LABEL: define void @affected_value_has_side_effect() { +; CHECK-NEXT: [[X:%.*]] = call i32 @get() +; CHECK-NEXT: ret void +; + %x = call i32 @get() + %cond = icmp sge i32 %x, 0 + call void @llvm.assume(i1 %cond) + ret void +} + +define i32 @affected_value_has_side_effect_and_is_used() { +; CHECK-LABEL: define i32 @affected_value_has_side_effect_and_is_used() { +; CHECK-NEXT: [[X:%.*]] = call i32 @get() +; CHECK-NEXT: [[COND:%.*]] = icmp sge i32 [[X]], 0 +; CHECK-NEXT: call void @llvm.assume(i1 [[COND]]) +; CHECK-NEXT: ret i32 [[X]] +; + %x = call i32 @get() + %cond = icmp sge i32 %x, 0 + call void @llvm.assume(i1 %cond) + ret i32 %x +} From f2094fe77a3325d04b1107fb72fc8422e9f9431c Mon Sep 17 00:00:00 2001 From: Nikita Popov Date: Fri, 26 Sep 2025 11:57:22 +0200 Subject: [PATCH 3/5] Add test showing inconsistent behavior with assume on global --- .../DropUnnecessaryAssumes/basic.ll | 27 +++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll b/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll index ad1912e0c4662..829a1ce76e8d9 100644 --- a/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll +++ b/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll @@ -287,3 +287,30 @@ define i32 @affected_value_has_side_effect_and_is_used() { call void @llvm.assume(i1 %cond) ret i32 %x } + +@g = external global i8 +@g2 = external global i8 + +define void @assume_on_global() { +; CHECK-LABEL: define void @assume_on_global() { +; CHECK-NEXT: ret void +; + call void @llvm.assume(i1 true) ["align"(ptr @g, i64 8)] + ret void +} + +define void @assume_on_global_used_in_other_func() { +; CHECK-LABEL: define void @assume_on_global_used_in_other_func() { +; CHECK-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr @g2, i64 8) ] +; CHECK-NEXT: ret void +; + call void @llvm.assume(i1 true) ["align"(ptr @g2, i64 8)] + ret void +} + +define ptr @other_func() { +; CHECK-LABEL: define ptr @other_func() { +; CHECK-NEXT: ret ptr @g2 +; + ret ptr @g2 +} From 3f2736c2d87bc678a5fe5efef4707e79ef19009b Mon Sep 17 00:00:00 2001 From: Nikita Popov Date: Fri, 26 Sep 2025 12:00:35 +0200 Subject: [PATCH 4/5] Don't handle globals Otherwise we get inconsistent behavior based on uses in other functions. This means we are also guaranteed that all users are Instructions. --- .../Transforms/Scalar/DropUnnecessaryAssumes.cpp | 16 ++++++++++------ .../Transforms/DropUnnecessaryAssumes/basic.ll | 2 ++ 2 files changed, 12 insertions(+), 6 deletions(-) diff --git a/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp b/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp index d4fb420211cb0..8539616484bc4 100644 --- a/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp +++ b/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp @@ -22,24 +22,28 @@ static bool affectedValuesAreEphemeral(ArrayRef Affected) { // 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 Worklist; + SmallSetVector Worklist; auto AddUser = [&](User *U) { // Bail out if we need to inspect too many users. if (Worklist.size() >= 32) return false; - Worklist.insert(U); + Worklist.insert(cast(U)); return true; }; - for (Value *V : Affected) + for (Value *V : Affected) { + // Do not handle assumes on globals for now. The use list for them may + // contain uses in other functions. + if (!isa(V)) + return false; + for (User *U : V->users()) if (!AddUser(U)) return false; + } for (unsigned Idx = 0; Idx < Worklist.size(); ++Idx) { - auto *I = dyn_cast(Worklist[Idx]); - if (!I) - return false; + Instruction *I = Worklist[Idx]; // Use in assume is ephemeral. if (isa(I)) diff --git a/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll b/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll index 829a1ce76e8d9..8a6f60ba7a204 100644 --- a/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll +++ b/llvm/test/Transforms/DropUnnecessaryAssumes/basic.ll @@ -291,8 +291,10 @@ define i32 @affected_value_has_side_effect_and_is_used() { @g = external global i8 @g2 = external global i8 +; Assumes on globals are currently not supported. define void @assume_on_global() { ; CHECK-LABEL: define void @assume_on_global() { +; CHECK-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr @g, i64 8) ] ; CHECK-NEXT: ret void ; call void @llvm.assume(i1 true) ["align"(ptr @g, i64 8)] From c4d9c7f4ad9791af02ad9d29ee5969bf73e1a069 Mon Sep 17 00:00:00 2001 From: Nikita Popov Date: Fri, 26 Sep 2025 12:03:11 +0200 Subject: [PATCH 5/5] Style tweaks --- .../Scalar/DropUnnecessaryAssumes.cpp | 24 +++++++++---------- 1 file changed, 12 insertions(+), 12 deletions(-) diff --git a/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp b/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp index 8539616484bc4..89980d54ee897 100644 --- a/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp +++ b/llvm/lib/Transforms/Scalar/DropUnnecessaryAssumes.cpp @@ -22,12 +22,14 @@ static bool affectedValuesAreEphemeral(ArrayRef Affected) { // 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 Worklist; - auto AddUser = [&](User *U) { - // Bail out if we need to inspect too many users. - if (Worklist.size() >= 32) - return false; - Worklist.insert(cast(U)); + SmallSetVector Worklist; + auto AddUsers = [&](Value *V) { + for (User *U : V->users()) { + // Bail out if we need to inspect too many users. + if (Worklist.size() >= 32) + return false; + Worklist.insert(cast(U)); + } return true; }; @@ -37,9 +39,8 @@ static bool affectedValuesAreEphemeral(ArrayRef Affected) { if (!isa(V)) return false; - for (User *U : V->users()) - if (!AddUser(U)) - return false; + if (!AddUsers(V)) + return false; } for (unsigned Idx = 0; Idx < Worklist.size(); ++Idx) { @@ -54,9 +55,8 @@ static bool affectedValuesAreEphemeral(ArrayRef Affected) { return false; // Otherwise, recursively look at the users. - for (User *NestedU : I->users()) - if (!AddUser(NestedU)) - return false; + if (!AddUsers(I)) + return false; } return true;