[llvm-branch-commits] [llvm] release/20.x: Do not treat llvm.fake.use as a debug instruction (#128684) (PR #128734)

[via llvm-branch-commits]

https://github.com/llvmbot updated https://github.com/llvm/llvm-project/pull/128734

>From addf1c97117e52bd52f445a360047c2752df9d1a Mon Sep 17 00:00:00 2001
From: Stephen Tozer <stephen.tozer at sony.com>
Date: Tue, 25 Feb 2025 14:49:59 +0000
Subject: [PATCH] Do not treat llvm.fake.use as a debug instruction (#128684)

The llvm.fake.use intrinsic is used to prevent certain values from being
optimized out for the benefit of debug info; it is not, however, a debug
or pseudo instruction itself and necessarily must not be treated as one,
since its purpose is to act like a normal instruction. In the original
commit that added them, the IR intrinsic however was treated as one in
`getPrevNonDebugInstruction` (but _not_ in `getNextNonDebugInstruction`,
or in the MIR equivalents). This patch correctly treats it as a
non-debug instruction.

(cherry picked from commit af68927a831c45b92248b1f6fc24d445be42dd91)
---
 llvm/lib/IR/Instruction.cpp                   |  5 +-
 .../X86/fake-use-considered-when-sinking.ll   | 67 +++++++++++++++++++
 2 files changed, 68 insertions(+), 4 deletions(-)
 create mode 100644 llvm/test/Transforms/SimplifyCFG/X86/fake-use-considered-when-sinking.ll

diff --git a/llvm/lib/IR/Instruction.cpp b/llvm/lib/IR/Instruction.cpp
index 84d9306ca6700..7d43de982df0d 100644
--- a/llvm/lib/IR/Instruction.cpp
+++ b/llvm/lib/IR/Instruction.cpp
@@ -1233,10 +1233,7 @@ Instruction::getNextNonDebugInstruction(bool SkipPseudoOp) const {
 const Instruction *
 Instruction::getPrevNonDebugInstruction(bool SkipPseudoOp) const {
   for (const Instruction *I = getPrevNode(); I; I = I->getPrevNode())
-    if (!isa<DbgInfoIntrinsic>(I) &&
-        !(SkipPseudoOp && isa<PseudoProbeInst>(I)) &&
-        !(isa<IntrinsicInst>(I) &&
-          cast<IntrinsicInst>(I)->getIntrinsicID() == Intrinsic::fake_use))
+    if (!isa<DbgInfoIntrinsic>(I) && !(SkipPseudoOp && isa<PseudoProbeInst>(I)))
       return I;
   return nullptr;
 }
diff --git a/llvm/test/Transforms/SimplifyCFG/X86/fake-use-considered-when-sinking.ll b/llvm/test/Transforms/SimplifyCFG/X86/fake-use-considered-when-sinking.ll
new file mode 100644
index 0000000000000..63217315c978c
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/X86/fake-use-considered-when-sinking.ll
@@ -0,0 +1,67 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -p="simplifycfg<sink-common-insts>" -S < %s | FileCheck %s
+
+;; Verify that fake uses are not ignored when sinking instructions in
+;; SimplifyCFG; when a fake use appears in only one incoming block they prevent
+;; further sinking, and when identical fake uses appear on both sides they
+;; are sunk normally.
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @foo(i1 %bool, ptr %p) {
+; CHECK-LABEL: define void @foo(
+; CHECK-SAME: i1 [[BOOL:%.*]], ptr [[P:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br i1 [[BOOL]], label %[[IF_ELSE:.*]], label %[[IF_THEN:.*]]
+; CHECK:       [[COMMON_RET:.*]]:
+; CHECK-NEXT:    ret void
+; CHECK:       [[IF_THEN]]:
+; CHECK-NEXT:    store ptr [[P]], ptr [[P]], align 8
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+; CHECK:       [[IF_ELSE]]:
+; CHECK-NEXT:    store ptr [[P]], ptr [[P]], align 8
+; CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[P]])
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+;
+entry:
+  br i1 %bool, label %if.else, label %if.then
+
+common.ret:                                       ; preds = %if.else, %if.then
+  ret void
+
+if.then:                                          ; preds = %entry
+  store ptr %p, ptr %p, align 8
+  br label %common.ret
+
+if.else:                                          ; preds = %entry
+  store ptr %p, ptr %p, align 8
+  notail call void (...) @llvm.fake.use(ptr %p)
+  br label %common.ret
+}
+
+define void @bar(i1 %bool, ptr %p) {
+; CHECK-LABEL: define void @bar(
+; CHECK-SAME: i1 [[BOOL:%.*]], ptr [[P:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    store ptr [[P]], ptr [[P]], align 8
+; CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[P]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 %bool, label %if.else, label %if.then
+
+common.ret:                                       ; preds = %if.else, %if.then
+  ret void
+
+if.then:                                          ; preds = %entry
+  store ptr %p, ptr %p, align 8
+  notail call void (...) @llvm.fake.use(ptr %p)
+  br label %common.ret
+
+if.else:                                          ; preds = %entry
+  store ptr %p, ptr %p, align 8
+  notail call void (...) @llvm.fake.use(ptr %p)
+  br label %common.ret
+}
+