[llvm] [InstCombine] Intersect nowrap flags between geps while folding into phi (PR #115901)

[Antonio Frighetto via llvm-commits]

https://github.com/antoniofrighetto updated https://github.com/llvm/llvm-project/pull/115901

>From 07384be091b6b334653ea8cd793286b7aec66545 Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Tue, 12 Nov 2024 10:45:46 +0100
Subject: [PATCH 1/2] [InstCombine] Precommit test for PR115901 (NFC)

---
 llvm/test/Transforms/InstCombine/phi.ll | 29 +++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/phi.ll b/llvm/test/Transforms/InstCombine/phi.ll
index 4e90a8f0ba4313..c726947318eb40 100644
--- a/llvm/test/Transforms/InstCombine/phi.ll
+++ b/llvm/test/Transforms/InstCombine/phi.ll
@@ -2794,3 +2794,32 @@ BB4:                                             ; preds = %BB3, %BB2, %BB1, %BB
 BB5:                                             ; preds = %BB4
   ret void
 }
+
+; FIXME: This is a miscompilation.
+define i64 @wrong_gep_arg_into_phi(ptr noundef %ptr) {
+; CHECK-LABEL: @wrong_gep_arg_into_phi(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    [[PTR_PN:%.*]] = phi ptr [ [[PTR:%.*]], [[ENTRY:%.*]] ], [ [[DOTPN:%.*]], [[FOR_COND]] ]
+; CHECK-NEXT:    [[DOTPN]] = getelementptr inbounds nuw i8, ptr [[PTR_PN]], i64 1
+; CHECK-NEXT:    [[VAL:%.*]] = load i8, ptr [[DOTPN]], align 1
+; CHECK-NEXT:    [[COND_NOT:%.*]] = icmp eq i8 [[VAL]], 0
+; CHECK-NEXT:    br i1 [[COND_NOT]], label [[EXIT:%.*]], label [[FOR_COND]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret i64 0
+;
+entry:
+  %add.ptr = getelementptr i8, ptr %ptr, i64 1
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.cond, %entry
+  %.pn = phi ptr [ %add.ptr, %entry ], [ %incdec.ptr, %for.cond ]
+  %val = load i8, ptr %.pn, align 1
+  %cond = icmp ne i8 %val, 0
+  %incdec.ptr = getelementptr inbounds nuw i8, ptr %.pn, i64 1
+  br i1 %cond, label %for.cond, label %exit
+
+exit:                                             ; preds = %for.cond
+  ret i64 0
+}

>From e5e171a032c9dbb700dc70fff14703cfc35d1c67 Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Tue, 12 Nov 2024 19:02:04 +0100
Subject: [PATCH 2/2] [InstCombine] Intersect nowrap flags between geps while
 folding into phi

A miscompilation issue has been addressed with refined checking.

Fixes: https://github.com/llvm/llvm-project/issues/115149.
---
 llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp | 3 ++-
 llvm/test/Transforms/InstCombine/opaque-ptr.ll     | 2 +-
 llvm/test/Transforms/InstCombine/phi.ll            | 3 +--
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp b/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
index cb5c4473051262..1fcf1c570addac 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
@@ -545,7 +545,8 @@ Instruction *InstCombinerImpl::foldPHIArgGEPIntoPHI(PHINode &PN) {
   // especially bad when the PHIs are in the header of a loop.
   bool NeededPhi = false;
 
-  GEPNoWrapFlags NW = GEPNoWrapFlags::all();
+  // Remember flags of the first phi-operand getelementptr.
+  GEPNoWrapFlags NW = FirstInst->getNoWrapFlags();
 
   // Scan to see if all operands are the same opcode, and all have one user.
   for (Value *V : drop_begin(PN.incoming_values())) {
diff --git a/llvm/test/Transforms/InstCombine/opaque-ptr.ll b/llvm/test/Transforms/InstCombine/opaque-ptr.ll
index a52a07389baee2..4e0355e61739a8 100644
--- a/llvm/test/Transforms/InstCombine/opaque-ptr.ll
+++ b/llvm/test/Transforms/InstCombine/opaque-ptr.ll
@@ -549,7 +549,7 @@ define ptr @phi_of_gep_flags_1(i1 %c, ptr %p) {
 ; CHECK:       else:
 ; CHECK-NEXT:    br label [[JOIN]]
 ; CHECK:       join:
-; CHECK-NEXT:    [[PHI:%.*]] = getelementptr nusw nuw i8, ptr [[P:%.*]], i64 4
+; CHECK-NEXT:    [[PHI:%.*]] = getelementptr nusw i8, ptr [[P:%.*]], i64 4
 ; CHECK-NEXT:    ret ptr [[PHI]]
 ;
   br i1 %c, label %if, label %else
diff --git a/llvm/test/Transforms/InstCombine/phi.ll b/llvm/test/Transforms/InstCombine/phi.ll
index c726947318eb40..c3548590d7f57a 100644
--- a/llvm/test/Transforms/InstCombine/phi.ll
+++ b/llvm/test/Transforms/InstCombine/phi.ll
@@ -2795,14 +2795,13 @@ BB5:                                             ; preds = %BB4
   ret void
 }
 
-; FIXME: This is a miscompilation.
 define i64 @wrong_gep_arg_into_phi(ptr noundef %ptr) {
 ; CHECK-LABEL: @wrong_gep_arg_into_phi(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[FOR_COND:%.*]]
 ; CHECK:       for.cond:
 ; CHECK-NEXT:    [[PTR_PN:%.*]] = phi ptr [ [[PTR:%.*]], [[ENTRY:%.*]] ], [ [[DOTPN:%.*]], [[FOR_COND]] ]
-; CHECK-NEXT:    [[DOTPN]] = getelementptr inbounds nuw i8, ptr [[PTR_PN]], i64 1
+; CHECK-NEXT:    [[DOTPN]] = getelementptr i8, ptr [[PTR_PN]], i64 1
 ; CHECK-NEXT:    [[VAL:%.*]] = load i8, ptr [[DOTPN]], align 1
 ; CHECK-NEXT:    [[COND_NOT:%.*]] = icmp eq i8 [[VAL]], 0
 ; CHECK-NEXT:    br i1 [[COND_NOT]], label [[EXIT:%.*]], label [[FOR_COND]]