[llvm] [DA] do not handle GEPs with different types (PR #144088)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: Sebastian Pop (sebpop)

<details>
<summary>Changes</summary>

This patch fixes an error in which the delinearized function for two GEPs end up to be the same except that the GEP type is different:
```
[256 x [256 x [256 x i64]]]
vs.
[256 x [256 x [256 x i32]]]
```
the result was that the dependence test was inferring a simple dependence with all elements overlapping, however because the type of elements indexed by the GEP are different, the overlap is non linear.  This patch filters out such cases.

---
Full diff: https://github.com/llvm/llvm-project/pull/144088.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/DependenceAnalysis.cpp (+10) 
- (modified) llvm/test/Analysis/DependenceAnalysis/DifferentOffsets.ll (+3-8) 


``````````diff
diff --git a/llvm/lib/Analysis/DependenceAnalysis.cpp b/llvm/lib/Analysis/DependenceAnalysis.cpp
index c1b1d002c9979..4c9ad4731b802 100644
--- a/llvm/lib/Analysis/DependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/DependenceAnalysis.cpp
@@ -3642,6 +3642,16 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
 
   Value *SrcPtr = getLoadStorePointerOperand(Src);
   Value *DstPtr = getLoadStorePointerOperand(Dst);
+
+  if (GetElementPtrInst *SrcGEP = dyn_cast<GetElementPtrInst>(SrcPtr)) {
+    if (GetElementPtrInst *DstGEP = dyn_cast<GetElementPtrInst>(DstPtr)) {
+      if (SrcGEP->getSourceElementType() != DstGEP->getSourceElementType()) {
+        LLVM_DEBUG(dbgs() << "can't analyze GEPs with different types\n");
+        return std::make_unique<Dependence>(Src, Dst,
+                                            SCEVUnionPredicate(Assume, *SE));
+      }
+    }
+  }
   const SCEV *SrcSCEV = SE->getSCEV(SrcPtr);
   const SCEV *DstSCEV = SE->getSCEV(DstPtr);
   LLVM_DEBUG(dbgs() << "    SrcSCEV = " << *SrcSCEV << "\n");
diff --git a/llvm/test/Analysis/DependenceAnalysis/DifferentOffsets.ll b/llvm/test/Analysis/DependenceAnalysis/DifferentOffsets.ll
index 1f8fac3087bff..5397fa5b97caf 100644
--- a/llvm/test/Analysis/DependenceAnalysis/DifferentOffsets.ll
+++ b/llvm/test/Analysis/DependenceAnalysis/DifferentOffsets.ll
@@ -76,13 +76,9 @@ define i32 @gep_i8_vs_i32(ptr nocapture %A, i64 %n, i64 %m) {
 ; CHECK-NEXT:  Src: store i32 42, ptr %arrayidx0, align 1 --> Dst: store i32 42, ptr %arrayidx0, align 1
 ; CHECK-NEXT:    da analyze - none!
 ; CHECK-NEXT:  Src: store i32 42, ptr %arrayidx0, align 1 --> Dst: store i32 42, ptr %arrayidx1, align 4
-; CHECK-NEXT:    da analyze - output [|<]!
-; CHECK-NEXT:    Runtime Assumptions:
-; CHECK-NEXT:    Equal predicate: (zext i2 (trunc i64 %n to i2) to i64) == 0
+; CHECK-NEXT:    da analyze - confused!
 ; CHECK-NEXT:  Src: store i32 42, ptr %arrayidx1, align 4 --> Dst: store i32 42, ptr %arrayidx1, align 4
 ; CHECK-NEXT:    da analyze - none!
-; CHECK-NEXT:  Runtime Assumptions:
-; CHECK-NEXT:  Equal predicate: (zext i2 (trunc i64 %n to i2) to i64) == 0
 ;
 entry:
   %arrayidx0 = getelementptr inbounds i8, ptr %A, i64 %n
@@ -98,7 +94,7 @@ define void @linearized_accesses(i64 %n, i64 %m, i64 %o, ptr %A) {
 ; CHECK-NEXT:  Src: store i32 1, ptr %idx0, align 4 --> Dst: store i32 1, ptr %idx0, align 4
 ; CHECK-NEXT:    da analyze - output [* * *]!
 ; CHECK-NEXT:  Src: store i32 1, ptr %idx0, align 4 --> Dst: store i32 1, ptr %idx1, align 4
-; CHECK-NEXT:    da analyze - output [* * *|<]!
+; CHECK-NEXT:    da analyze - confused!
 ; CHECK-NEXT:  Src: store i32 1, ptr %idx1, align 4 --> Dst: store i32 1, ptr %idx1, align 4
 ; CHECK-NEXT:    da analyze - none!
 ;
@@ -149,8 +145,7 @@ define void @multidim_accesses(ptr %A) {
 ; CHECK-NEXT:  Src: store i32 1, ptr %idx0, align 4 --> Dst: store i32 1, ptr %idx0, align 4
 ; CHECK-NEXT:    da analyze - none!
 ; CHECK-NEXT:  Src: store i32 1, ptr %idx0, align 4 --> Dst: store i32 1, ptr %idx1, align 4
-; FIXME: the dependence distance is not constant. Distance vector should be [* * *|<]!
-; CHECK-NEXT:    da analyze - consistent output [0 0 0|<]!
+; CHECK-NEXT:    da analyze - confused!
 ; CHECK-NEXT:  Src: store i32 1, ptr %idx1, align 4 --> Dst: store i32 1, ptr %idx1, align 4
 ; CHECK-NEXT:    da analyze - none!
 ;

``````````

</details>


https://github.com/llvm/llvm-project/pull/144088