[PATCH] D103874: [IR] Rename the shufflevector's undef mask to poison

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

I noted the cases where it looks like the undef->poison change might actually impact code using compiler intrinsic functions that have external specifications.  The relevant specifications say the elements in question are "undefined", without really specifying what that means.

Currently, for the Intel intrinsics, we treat "undefined" as something more conservative than LLVM undef; see https://github.com/llvm/llvm-project/blob/d2012d965d60c3258b3a69d024491698f8aec386/clang/lib/CodeGen/CGBuiltin.cpp#L12483 .  Maybe we should make the cast intrinsics more conservative to match.  And maybe we should do the same for OpenCL.  Would need to do some backend work to make sure we don't regress the generated code, though.

For __builtin_shufflevector, I think I'm fine with this changing the "-1" to mean poison; we don't have any external spec to conform to, and anyone explicitly passing -1 should know what they're doing.  But maybe worth noting in the clang documentation.



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Comment at: clang/test/CodeGen/X86/avx-builtins.c:182
   // CHECK-LABEL: test_mm256_castsi128_si256
-  // CHECK: shufflevector <2 x i64> %{{.*}}, <2 x i64> %{{.*}}, <4 x i32> <i32 0, i32 1, i32 undef, i32 undef>
+  // CHECK: shufflevector <2 x i64> %{{.*}}, <2 x i64> %{{.*}}, <4 x i32> <i32 0, i32 1, i32 poison, i32 poison>
   return _mm256_castsi128_si256(A);
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This change might be visible to user code.


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Comment at: clang/test/CodeGen/builtinshufflevector2.c:41
 // CHECK: store <4 x float> [[V]], <4 x float>* {{%.*}}
   *A = __builtin_shufflevector( x, y, 0, 4, -1, 5 );
 }
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This might be visible to user code.


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Comment at: clang/test/CodeGenOpenCL/preserve_vec3.cl:21
   // CHECK: %[[LOAD_A:.*]] = load <3 x float>, <3 x float> addrspace(1)* %a, align 16
-  // CHECK: %[[ASTYPE:.*]] = shufflevector <3 x float> %[[LOAD_A]], <3 x float> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 undef>
+  // CHECK: %[[ASTYPE:.*]] = shufflevector <3 x float> %[[LOAD_A]], <3 x float> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
   // CHECK: store <4 x float> %[[ASTYPE:.*]], <4 x float> addrspace(1)* %b, align 16
----------------
This change might be visible to user code.


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  https://reviews.llvm.org/D103874/new/

https://reviews.llvm.org/D103874