[llvm] [DAG] Assure that ComputeNumSignBits returns 1 (PR #155455)

[Miguel Saldivar via llvm-commits]

https://github.com/Saldivarcher updated https://github.com/llvm/llvm-project/pull/155455

>From f3e7c61917e30188f5704d759d5345808c5d7a88 Mon Sep 17 00:00:00 2001
From: Miguel Saldivar <miguel.saldivar at hpe.com>
Date: Tue, 26 Aug 2025 12:43:05 -0500
Subject: [PATCH] [DAG] Assure that ComputeNumSignBits returns 1

If the value for `KnownSign - rIndex * BitWidth` produces a negative,
we need `ComputeNumSignBits` to return a 1. Changing the `std::clamp`
lower bound value should fix that.

This is a fix for #155452.
---
 .../lib/CodeGen/SelectionDAG/SelectionDAG.cpp |  2 +-
 llvm/test/CodeGen/AMDGPU/pr155452.ll          | 84 +++++++++++++++++++
 2 files changed, 85 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/CodeGen/AMDGPU/pr155452.ll

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 3672a91e33a30..078825f2a9a22 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5127,7 +5127,7 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
 
     // If the sign portion ends in our element the subtraction gives correct
     // result. Otherwise it gives either negative or > bitwidth result
-    return std::clamp(KnownSign - rIndex * BitWidth, 0, BitWidth);
+    return std::clamp(KnownSign - rIndex * BitWidth, 1, BitWidth);
   }
   case ISD::INSERT_VECTOR_ELT: {
     if (VT.isScalableVector())
diff --git a/llvm/test/CodeGen/AMDGPU/pr155452.ll b/llvm/test/CodeGen/AMDGPU/pr155452.ll
new file mode 100644
index 0000000000000..d021b210c7f3a
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/pr155452.ll
@@ -0,0 +1,84 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc %s -march=amdgcn  -o - | FileCheck %s
+
+target triple = "amdgcn-amd-amdhsa"
+
+define amdgpu_kernel void @my_kernel(i64 %foo, i32 %bar) {
+; CHECK-LABEL: my_kernel:
+; CHECK:       ; %bb.0: ; %entry
+; CHECK-NEXT:    s_mov_b32 flat_scratch_lo, s13
+; CHECK-NEXT:    s_add_i32 s12, s12, s17
+; CHECK-NEXT:    s_lshr_b32 flat_scratch_hi, s12, 8
+; CHECK-NEXT:    s_load_dword s0, s[8:9], 0x2
+; CHECK-NEXT:    s_load_dwordx2 s[2:3], s[8:9], 0x0
+; CHECK-NEXT:    s_mov_b64 s[4:5], 1
+; CHECK-NEXT:    v_mov_b32_e32 v0, 0
+; CHECK-NEXT:    v_mov_b32_e32 v1, 0
+; CHECK-NEXT:    s_waitcnt lgkmcnt(0)
+; CHECK-NEXT:    s_ashr_i32 s6, s0, 31
+; CHECK-NEXT:    s_abs_i32 s7, s0
+; CHECK-NEXT:    v_cvt_f32_u32_e32 v2, s7
+; CHECK-NEXT:    s_sub_i32 s0, 0, s7
+; CHECK-NEXT:    v_rcp_iflag_f32_e32 v2, v2
+; CHECK-NEXT:    v_mul_f32_e32 v2, 0x4f7ffffe, v2
+; CHECK-NEXT:    v_cvt_u32_f32_e32 v2, v2
+; CHECK-NEXT:    v_mul_lo_u32 v3, s0, v2
+; CHECK-NEXT:    v_mul_hi_u32 v3, v2, v3
+; CHECK-NEXT:    v_add_i32_e32 v2, vcc, v2, v3
+; CHECK-NEXT:    s_and_b64 s[0:1], exec, -1
+; CHECK-NEXT:  .LBB0_1: ; %loop
+; CHECK-NEXT:    ; =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    v_mov_b32_e32 v3, s4
+; CHECK-NEXT:    v_mad_u64_u32 v[3:4], s[8:9], s2, v3, 1
+; CHECK-NEXT:    s_mul_i32 s4, s3, s4
+; CHECK-NEXT:    s_mul_i32 s5, s2, s5
+; CHECK-NEXT:    v_add_i32_e32 v4, vcc, s4, v4
+; CHECK-NEXT:    v_readfirstlane_b32 s4, v3
+; CHECK-NEXT:    v_add_i32_e32 v4, vcc, s5, v4
+; CHECK-NEXT:    s_ashr_i32 s5, s4, 31
+; CHECK-NEXT:    s_abs_i32 s8, s4
+; CHECK-NEXT:    s_xor_b32 s5, s5, s6
+; CHECK-NEXT:    v_mul_hi_u32 v3, s8, v2
+; CHECK-NEXT:    v_readfirstlane_b32 s9, v3
+; CHECK-NEXT:    v_add_i32_e32 v5, vcc, 1, v3
+; CHECK-NEXT:    s_mul_i32 s9, s9, s7
+; CHECK-NEXT:    s_sub_i32 s8, s8, s9
+; CHECK-NEXT:    s_sub_i32 s9, s8, s7
+; CHECK-NEXT:    s_cmp_ge_u32 s8, s7
+; CHECK-NEXT:    s_cselect_b64 vcc, -1, 0
+; CHECK-NEXT:    v_cndmask_b32_e32 v3, v3, v5, vcc
+; CHECK-NEXT:    s_cselect_b32 s8, s9, s8
+; CHECK-NEXT:    v_add_i32_e32 v5, vcc, 1, v3
+; CHECK-NEXT:    s_cmp_ge_u32 s8, s7
+; CHECK-NEXT:    s_cselect_b64 vcc, -1, 0
+; CHECK-NEXT:    v_cndmask_b32_e32 v3, v3, v5, vcc
+; CHECK-NEXT:    v_xor_b32_e32 v3, s5, v3
+; CHECK-NEXT:    v_subrev_i32_e32 v3, vcc, s5, v3
+; CHECK-NEXT:    v_ashrrev_i32_e32 v5, 31, v3
+; CHECK-NEXT:    v_or_b32_e32 v3, s4, v3
+; CHECK-NEXT:    v_or_b32_e32 v4, v4, v5
+; CHECK-NEXT:    flat_load_dwordx2 v[3:4], v[3:4]
+; CHECK-NEXT:    s_waitcnt vmcnt(0)
+; CHECK-NEXT:    flat_store_dwordx2 v[0:1], v[3:4]
+; CHECK-NEXT:    s_mov_b64 s[4:5], 0
+; CHECK-NEXT:    s_mov_b64 vcc, s[0:1]
+; CHECK-NEXT:    s_cbranch_vccnz .LBB0_1
+; CHECK-NEXT:  ; %bb.2: ; %DummyReturnBlock
+; CHECK-NEXT:    s_endpgm
+entry:
+  br label %loop
+
+loop: ; preds = %entry, %loop
+  %i = phi i64 [ 1, %entry ], [ 0, %loop ]
+  %mul = mul i64 %foo, %i
+  %add = add i64 %mul, 1
+  %trunc = trunc i64 %add to i32
+  %div = sdiv i32 %trunc, %bar
+  %sext = sext i32 %div to i64
+  %or = or i64 %add, %sext
+  %inttoptr = inttoptr i64 %or to ptr
+  %addrspacecast = addrspacecast ptr %inttoptr to ptr addrspace(1)
+  %val = load double, ptr addrspace(1) %addrspacecast, align 8
+  store double %val, ptr addrspace(1) null, align 8
+  br label %loop
+}