[llvm] [InstCombine] Do not keep samesign when speculatively executing icmps (PR #127007)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/127007

Closes https://github.com/llvm/llvm-project/issues/126974.


>From 1b100ae4fc11b435a46e5680e4157fe3ca907ed6 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Thu, 13 Feb 2025 11:58:31 +0800
Subject: [PATCH 1/2] [InstCombine] Add pre-commit tests. NFC.

---
 llvm/test/Transforms/InstCombine/umax-icmp.ll | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/umax-icmp.ll b/llvm/test/Transforms/InstCombine/umax-icmp.ll
index b4eea30bfc6af..87f4322e8e100 100644
--- a/llvm/test/Transforms/InstCombine/umax-icmp.ll
+++ b/llvm/test/Transforms/InstCombine/umax-icmp.ll
@@ -804,4 +804,27 @@ end:
   ret void
 }
 
+define i1 @pr126974(i8 %x) {
+; CHECK-LABEL: @pr126974(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[COND:%.*]] = icmp sgt i8 [[X:%.*]], -2
+; CHECK-NEXT:    br i1 [[COND]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    ret i1 false
+; CHECK:       if.else:
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %cond = icmp sgt i8 %x, -2
+  br i1 %cond, label %if.then, label %if.else
+
+if.then:
+  %umax = call i8 @llvm.umax.i8(i8 %x, i8 -46)
+  %cmp = icmp samesign ult i8 %umax, -32
+  ret i1 %cmp
+
+if.else:
+  ret i1 false
+}
+
 declare i32 @llvm.umax.i32(i32, i32)

>From 0c2b9c637b2033dd147b004a0c780d3dc5227d70 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Thu, 13 Feb 2025 12:00:48 +0800
Subject: [PATCH 2/2] [InstCombine] Do not keep samesign when speculatively
 executing icmps

---
 llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp | 5 +++++
 llvm/test/Transforms/InstCombine/umax-icmp.ll           | 3 ++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 0feb6160b68fb..00a8117f32e70 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -5636,6 +5636,11 @@ Instruction *InstCombinerImpl::foldICmpWithMinMax(Instruction &I,
       return false;
     return std::nullopt;
   };
+  // Remove samesign here since it is illegal to keep it when we speculatively
+  // execute comparisons. For example, `icmp samesign ult umax(X, -46), -32`
+  // cannot be decomposed into `(icmp samesign ult X, -46) or (icmp samesign ult
+  // -46, -32)`. `X` is allowed to be non-negative here.
+  Pred = static_cast<CmpInst::Predicate>(Pred);
   auto CmpXZ = IsCondKnownTrue(simplifyICmpInst(Pred, X, Z, Q));
   auto CmpYZ = IsCondKnownTrue(simplifyICmpInst(Pred, Y, Z, Q));
   if (!CmpXZ.has_value() && !CmpYZ.has_value())
diff --git a/llvm/test/Transforms/InstCombine/umax-icmp.ll b/llvm/test/Transforms/InstCombine/umax-icmp.ll
index 87f4322e8e100..0c42d26750e4b 100644
--- a/llvm/test/Transforms/InstCombine/umax-icmp.ll
+++ b/llvm/test/Transforms/InstCombine/umax-icmp.ll
@@ -810,7 +810,8 @@ define i1 @pr126974(i8 %x) {
 ; CHECK-NEXT:    [[COND:%.*]] = icmp sgt i8 [[X:%.*]], -2
 ; CHECK-NEXT:    br i1 [[COND]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[X]], -1
+; CHECK-NEXT:    ret i1 [[CMP]]
 ; CHECK:       if.else:
 ; CHECK-NEXT:    ret i1 false
 ;