[llvm-branch-commits] [llvm] [ValueTracking] Mark mask.beforefirst as propagating poison (PR #234473)

[Luke Lau via llvm-branch-commits]

https://github.com/lukel97 updated https://github.com/llvm/llvm-project/pull/234473

>From 0e9c418adef39ffb0781bb648d7d504db66fcc1d Mon Sep 17 00:00:00 2001
From: Luke Lau <luke at igalia.com>
Date: Thu, 22 Oct 2026 11:19:13 +0800
Subject: [PATCH 1/2] Precommit tests

---
 .../InstCombine/mask-beforefirst.ll           | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)
 create mode 100644 llvm/test/Transforms/InstCombine/mask-beforefirst.ll

diff --git a/llvm/test/Transforms/InstCombine/mask-beforefirst.ll b/llvm/test/Transforms/InstCombine/mask-beforefirst.ll
new file mode 100644
index 0000000000000..9868f89adcaff
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/mask-beforefirst.ll
@@ -0,0 +1,26 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -p instcombine -S < %s | FileCheck %s
+
+define <4 x i1> @select_or(<4 x i1> %m) {
+; CHECK-LABEL: define <4 x i1> @select_or(
+; CHECK-SAME: <4 x i1> [[M:%.*]]) {
+; CHECK-NEXT:    [[R:%.*]] = call <4 x i1> @llvm.mask.beforefirst.v4i1(<4 x i1> [[M]])
+; CHECK-NEXT:    [[SEL:%.*]] = select <4 x i1> [[R]], <4 x i1> splat (i1 true), <4 x i1> [[M]]
+; CHECK-NEXT:    ret <4 x i1> [[SEL]]
+;
+  %r = call <4 x i1> @llvm.mask.beforefirst(<4 x i1> %m)
+  %sel = select <4 x i1> %r, <4 x i1> splat (i1 true), <4 x i1> %m
+  ret <4 x i1> %sel
+}
+
+define <4 x i1> @select_and(<4 x i1> %m) {
+; CHECK-LABEL: define <4 x i1> @select_and(
+; CHECK-SAME: <4 x i1> [[M:%.*]]) {
+; CHECK-NEXT:    [[R:%.*]] = call <4 x i1> @llvm.mask.beforefirst.v4i1(<4 x i1> [[M]])
+; CHECK-NEXT:    [[SEL:%.*]] = select <4 x i1> [[R]], <4 x i1> [[M]], <4 x i1> zeroinitializer
+; CHECK-NEXT:    ret <4 x i1> [[SEL]]
+;
+  %r = call <4 x i1> @llvm.mask.beforefirst(<4 x i1> %m)
+  %sel = select <4 x i1> %r, <4 x i1> %m, <4 x i1> zeroinitializer
+  ret <4 x i1> %sel
+}

>From 011010701a928efb411bd0ea72516f40d23ba520 Mon Sep 17 00:00:00 2001
From: Luke Lau <luke at igalia.com>
Date: Thu, 22 Oct 2026 11:19:36 +0800
Subject: [PATCH 2/2] [ValueTracking] Mark mask.beforefirst as propagating
 poison

This allows us to combine some logical ands/ors to regular ands/ors.
---
 llvm/lib/Analysis/ValueTracking.cpp                  | 1 +
 llvm/test/Transforms/InstCombine/mask-beforefirst.ll | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 0d47b655f3a91..06db47739a268 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -8654,6 +8654,7 @@ bool llvm::intrinsicPropagatesPoison(Intrinsic::ID IID) {
   case Intrinsic::fshr:
   case Intrinsic::frexp:
   case Intrinsic::get_active_lane_mask:
+  case Intrinsic::mask_beforefirst:
     return true;
   default:
     return false;
diff --git a/llvm/test/Transforms/InstCombine/mask-beforefirst.ll b/llvm/test/Transforms/InstCombine/mask-beforefirst.ll
index 9868f89adcaff..13aacf3839c5e 100644
--- a/llvm/test/Transforms/InstCombine/mask-beforefirst.ll
+++ b/llvm/test/Transforms/InstCombine/mask-beforefirst.ll
@@ -5,7 +5,7 @@ define <4 x i1> @select_or(<4 x i1> %m) {
 ; CHECK-LABEL: define <4 x i1> @select_or(
 ; CHECK-SAME: <4 x i1> [[M:%.*]]) {
 ; CHECK-NEXT:    [[R:%.*]] = call <4 x i1> @llvm.mask.beforefirst.v4i1(<4 x i1> [[M]])
-; CHECK-NEXT:    [[SEL:%.*]] = select <4 x i1> [[R]], <4 x i1> splat (i1 true), <4 x i1> [[M]]
+; CHECK-NEXT:    [[SEL:%.*]] = or <4 x i1> [[R]], [[M]]
 ; CHECK-NEXT:    ret <4 x i1> [[SEL]]
 ;
   %r = call <4 x i1> @llvm.mask.beforefirst(<4 x i1> %m)
@@ -17,7 +17,7 @@ define <4 x i1> @select_and(<4 x i1> %m) {
 ; CHECK-LABEL: define <4 x i1> @select_and(
 ; CHECK-SAME: <4 x i1> [[M:%.*]]) {
 ; CHECK-NEXT:    [[R:%.*]] = call <4 x i1> @llvm.mask.beforefirst.v4i1(<4 x i1> [[M]])
-; CHECK-NEXT:    [[SEL:%.*]] = select <4 x i1> [[R]], <4 x i1> [[M]], <4 x i1> zeroinitializer
+; CHECK-NEXT:    [[SEL:%.*]] = and <4 x i1> [[R]], [[M]]
 ; CHECK-NEXT:    ret <4 x i1> [[SEL]]
 ;
   %r = call <4 x i1> @llvm.mask.beforefirst(<4 x i1> %m)