[clang] [clang] Implement constexpr bit_cast for vectors (PR #66894)

via cfe-commits cfe-commits at lists.llvm.org
Fri Sep 29 05:40:18 PDT 2023


================
@@ -7095,6 +7157,22 @@ class APValueToBufferConverter {
     return true;
   }
 
+  bool visitVector(const APValue &Val, QualType Ty, CharUnits Offset) {
+    const VectorType *VTy = Ty->castAs<VectorType>();
+
+    APInt Bits;
+    if (!BitcastVectorToAPInt(Info, VTy, Val, Bits))
+      return false;
+
+    assert(Bits.getBitWidth() == Info.Ctx.getTypeSize(Ty));
+
+    unsigned Width = Bits.getBitWidth();
+    SmallVector<uint8_t, 8> Bytes(Width / 8);
+    llvm::StoreIntToMemory(Bits, &*Bytes.begin(), Width / 8);
+    Buffer.writeObject(Offset, Bytes);
----------------
DaMatrix wrote:

This example on [Compiler Explorer](https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGe1wAyeAyYAHI%2BAEaYxCBmpAAOqAqETgwe3r56icmOAkEh4SxRMXF2mA6pQgRMxATpPn5ctpj2uQxVNQT5YZHRsbbVtfWZTQqDXcE9RX1mAJS2qF7EyOwcaAxjANS0AsCb6IsR9JtU8QAcGgD6CqIGxJsmAMwAIpsQO0b7h/Szm1ybAHo3h89gcvEdML9Hk8rBoAIImeFeZKfU4XS4AN3KDxe212X3Bx0ulyYBAIxDwES8BEwxKgWIcJGueAAXpgII85rNZjDEXC0VcGZtgJgCJjyhBfiYAOywuGbBWbYiipYME7nQXlGUWdXom5iGqkXVXfV3GXPXnw818vljYheBzG67kh0EB6yvGfMEQzZMI0RGHui2POUC532x0isV212S91yxVKlXENXap2mw3p241IOWhHSi3wjjzWicACsvD8HC0pFQnDc1msmwUi2WmAeZkePFIBE0xfmAGsQI9HgA6EcTydTgBs%2Bk4kkrfdrnF4ChAGh7ffmcFgSDQLHidGi5Eo%2B8P9BiyAMRkuLoYA9IWAxeBWADU8JgAO4AeXijE43Y0LQNLEGuEAREuETBDUACeAG8FBzDEDB34RNo5S9twvD7mwgjfgwtBwdWvBYFSwBuGItBrlhj6YCwhjAOIxGPngyoVFi1E1pgqjlNSqzdsENKlsxtCUsQsEeFgS7kngLDwfMVAGMACjvl%2Bv7/jR/CCCIYjsFIMiCIoKjqMxuhNNexiNpY%2BiUmukDzKg8RtNRvCoFixAUlgdmSs0rSpC4DDuJ4DT%2BIF3SFMUWRJCkAjDI0CTRW04W9DEowtBhlTjHFehlBUAgdLUyXTKlAydNlozjEVkVcPMLZLCsEgluWi7MXWHCbKoZzTgAtNOkibFeDFvHeA6/BAuCECQHZdrMvCYVo3KkEOI7jlOa0jrOwkLqQclcBoG5VjWbWruum7EduMCICgqAHkeZAUBAZ53SAynMPECgIKgBC0c%2Bb4fj%2Bf5VoBdAgWBEHMYhsHwaQkPIah6EONDOGMAQ%2BGEUupFeORlHUd2WD0UYTE1vgbGOBxS7cbxNLQ4JLRLqJETichkmrDWMlyVhClKSp/3qUDvBacItx6dIgtGWoS66HEFkoFZNgM95DlOakLm1u5nmYIrvkZc4ECuOVpCBJMEV9E02QxWkwUjAlOSpFVpva3l7RZVb8W5W0BUTAUKU5S7GRu5Vxs%2BzVCz1XpTUcBWpCHa5nAdV1vX9cAyDIMN9r3mNE1EPc5gzXNW6DsOY7retc4cNtMfLhwJ0bvN/Zl2YLVHSuZ0LfM7nJM4khAA%3D%3D%3D) seems to show that the padding bits are uninitialized for both vectors and scalars: in either case, both clang and gcc don't initialize the 6 bytes of padding after each fp80 element.

https://github.com/llvm/llvm-project/pull/66894


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