[llvm] [DAG] Improve simplifyDivRem's handling of zero numerators (PR #163531)

[Hari Limaye via llvm-commits]

https://github.com/hazzlim created https://github.com/llvm/llvm-project/pull/163531

Post legalization, simplifyDivRem will not work as well when folding
zero numerators. This is due to how we disallow truncation when calling
isConstOrConstSplat, similar to PR162706.

Fix this by refactoring the code to use `isZeroOrZeroSplat`, which
allows truncation by default.



>From 71472977bf2861a283ab388c21513435f268ca72 Mon Sep 17 00:00:00 2001
From: Hari Limaye <hari.limaye at arm.com>
Date: Thu, 9 Oct 2025 19:02:49 +0000
Subject: [PATCH 1/2] [DAG] Precommit test for 'Improve simplifyDivRem's
 handling of zero numerators' (NFC)

---
 llvm/test/CodeGen/AArch64/combine-sdiv.ll | 40 +++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/llvm/test/CodeGen/AArch64/combine-sdiv.ll b/llvm/test/CodeGen/AArch64/combine-sdiv.ll
index cca190f08df2b..442fc1bb15e7f 100644
--- a/llvm/test/CodeGen/AArch64/combine-sdiv.ll
+++ b/llvm/test/CodeGen/AArch64/combine-sdiv.ll
@@ -1859,3 +1859,43 @@ define <16 x i16> @combine_vec_sdiv_by_one_obfuscated(<16 x i16> %x) "target-fea
   %div = sdiv <16 x i16> %x, %zero_and_ones
   ret <16 x i16> %div
 }
+
+define <8 x i32> @combine_vec_sdiv_zero_obfuscated(<8 x i32> %x) {
+; CHECK-SD-LABEL: combine_vec_sdiv_zero_obfuscated:
+; CHECK-SD:       // %bb.0:
+; CHECK-SD-NEXT:    fmov w10, s0
+; CHECK-SD-NEXT:    mov w8, #1 // =0x1
+; CHECK-SD-NEXT:    mov w9, v0.s[1]
+; CHECK-SD-NEXT:    mov w11, v0.s[2]
+; CHECK-SD-NEXT:    mov w12, v0.s[3]
+; CHECK-SD-NEXT:    movi v1.2d, #0000000000000000
+; CHECK-SD-NEXT:    sdiv w10, w8, w10
+; CHECK-SD-NEXT:    sdiv w9, w8, w9
+; CHECK-SD-NEXT:    fmov s0, w10
+; CHECK-SD-NEXT:    sdiv w11, w8, w11
+; CHECK-SD-NEXT:    mov v0.s[1], w9
+; CHECK-SD-NEXT:    sdiv w8, w8, w12
+; CHECK-SD-NEXT:    mov v0.s[2], w11
+; CHECK-SD-NEXT:    mov v0.s[3], w8
+; CHECK-SD-NEXT:    ret
+;
+; CHECK-GI-LABEL: combine_vec_sdiv_zero_obfuscated:
+; CHECK-GI:       // %bb.0:
+; CHECK-GI-NEXT:    fmov w9, s0
+; CHECK-GI-NEXT:    mov w8, #1 // =0x1
+; CHECK-GI-NEXT:    mov w10, v0.s[1]
+; CHECK-GI-NEXT:    mov w11, v0.s[2]
+; CHECK-GI-NEXT:    mov w12, v0.s[3]
+; CHECK-GI-NEXT:    movi v1.2d, #0000000000000000
+; CHECK-GI-NEXT:    sdiv w9, w8, w9
+; CHECK-GI-NEXT:    sdiv w10, w8, w10
+; CHECK-GI-NEXT:    fmov s0, w9
+; CHECK-GI-NEXT:    sdiv w11, w8, w11
+; CHECK-GI-NEXT:    mov v0.s[1], w10
+; CHECK-GI-NEXT:    sdiv w8, w8, w12
+; CHECK-GI-NEXT:    mov v0.s[2], w11
+; CHECK-GI-NEXT:    mov v0.s[3], w8
+; CHECK-GI-NEXT:    ret
+  %1 = sdiv <8 x i32> <i32 1, i32 1, i32 1, i32 1, i32 0, i32 0, i32 0, i32 0>, %x
+  ret <8 x i32> %1
+}

>From dfe87df82b3ff17916c9c39d6ea9995846a05ad3 Mon Sep 17 00:00:00 2001
From: Hari Limaye <hari.limaye at arm.com>
Date: Thu, 9 Oct 2025 19:05:57 +0000
Subject: [PATCH 2/2] [DAG] Improve simplifyDivRem's handling of zero
 numerators

Post legalization, simplifyDivRem will not work as well when folding
zero numerators. This is due to how we disallow truncation when calling
isConstOrConstSplat, similar to PR162706.

Fix this by refactoring the code to use `isZeroOrZeroSplat`, which
allows truncation by default.
---
 llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 358e060d2c6d3..30421f7173211 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5060,8 +5060,7 @@ static SDValue simplifyDivRem(SDNode *N, SelectionDAG &DAG) {
 
   // 0 / X -> 0
   // 0 % X -> 0
-  ConstantSDNode *N0C = isConstOrConstSplat(N0);
-  if (N0C && N0C->isZero())
+  if (isZeroOrZeroSplat(N0))
     return N0;
 
   // X / X -> 1