[llvm] [X86] combineTruncate - trunc(srl(load(p),amt)) -> load(p+amt/8) - ensure there isn't an interdependency between the load and amt (PR #165850)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon created https://github.com/llvm/llvm-project/pull/165850

Fixes #165755

>From 183926d0a2b8df8a5ee72be45b2542ac240a0f43 Mon Sep 17 00:00:00 2001
From: Simon Pilgrim <llvm-dev at redking.me.uk>
Date: Fri, 31 Oct 2025 10:51:18 +0000
Subject: [PATCH] [X86] combineTruncate - trunc(srl(load(p),amt)) ->
 load(p+amt/8) - ensure there isn't an interdependency between the load and
 amt

Fixes #165755
---
 llvm/lib/Target/X86/X86ISelLowering.cpp |  4 +++-
 llvm/test/CodeGen/X86/pr165755.ll       | 26 +++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/CodeGen/X86/pr165755.ll

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 49beadae63f03..9525e03baa167 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -54768,9 +54768,11 @@ static SDValue combineTruncate(SDNode *N, SelectionDAG &DAG,
       KnownBits KnownAmt = DAG.computeKnownBits(ShAmt);
       // Check the shift amount is byte aligned.
       // Check the truncation doesn't use any shifted in (zero) top bits.
+      // Check the shift amount doesn't depend on the original load.
       if (KnownAmt.countMinTrailingZeros() >= 3 &&
           KnownAmt.getMaxValue().ule(SrcVT.getSizeInBits() -
-                                     VT.getSizeInBits())) {
+                                     VT.getSizeInBits()) &&
+          !Ld->isPredecessorOf(ShAmt.getNode())) {
         EVT PtrVT = Ld->getBasePtr().getValueType();
         SDValue PtrBitOfs = DAG.getZExtOrTrunc(ShAmt, DL, PtrVT);
         SDValue PtrByteOfs =
diff --git a/llvm/test/CodeGen/X86/pr165755.ll b/llvm/test/CodeGen/X86/pr165755.ll
new file mode 100644
index 0000000000000..3ab484f676c45
--- /dev/null
+++ b/llvm/test/CodeGen/X86/pr165755.ll
@@ -0,0 +1,26 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc < %s -mtriple=i686-- | FileCheck %s --check-prefixes=X86
+; RUN: llc < %s -mtriple=x86_64-- | FileCheck %s --check-prefixes=X64
+
+define i32 @PR165755(ptr %p0) {
+; X86-LABEL: PR165755:
+; X86:       # %bb.0:
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %ecx
+; X86-NEXT:    movl (%ecx), %eax
+; X86-NEXT:    movb $0, (%ecx)
+; X86-NEXT:    retl
+;
+; X64-LABEL: PR165755:
+; X64:       # %bb.0:
+; X64-NEXT:    movl (%rdi), %eax
+; X64-NEXT:    movb $0, (%rdi)
+; X64-NEXT:    retq
+  %ld64 = load i64, ptr %p0, align 8
+  store i8 0, ptr %p0, align 1
+  %ld32 = load i32, ptr %p0, align 8
+  %mask = and i32 %ld32, 32
+  %zext = zext i32 %mask to i64
+  %srl = lshr i64 %ld64, %zext
+  %res = trunc i64 %srl to i32
+  ret i32 %res
+}