[PATCH] D136532: [clang][Interp] Implement left and right shifts

[Shafik Yaghmour via Phabricator via cfe-commits]

shafik added inline comments.


================
Comment at: clang/test/AST/Interp/shifts.cpp:57
+    //c >>= 999999; // expected-warning {{shift count >= width of type}}
+    //c <<= CHAR_BIT; // expected-warning {{shift count >= width of type}}
+    //c >>= CHAR_BIT; // expected-warning {{shift count >= width of type}}
----------------
This is not correct, the operands go through integral promotions first and the result is the promoted type of the left operand see [dcl.shift p1](https://eel.is/c++draft/expr.shift#1).

Also see godbolt: https://godbolt.org/z/7qzKjojMb


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Comment at: clang/test/AST/Interp/shifts.cpp:70
+    i = 1 << (WORD_BIT - 1); // cxx17-warning-not {{sets the sign bit of the shift expression}}
+    i = -1 << (WORD_BIT - 1); // cxx17-warning {{shifting a negative signed value is undefined}} \
+                              // ref-cxx17-warning {{shifting a negative signed value is undefined}}
----------------
A negative left operand was made well-formed in C++20 I believe see godbolt: https://godbolt.org/z/7qzKjojMb

My reference from above for `expr.shift/p1` also applies. 

Although a negative right operand is still UB.

Also note shifting into the sign bit was made well-formed in C++11: https://stackoverflow.com/questions/19593938/is-left-shifting-a-negative-integer-undefined-behavior-in-c11#comment29091986_19593938


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D136532/new/

https://reviews.llvm.org/D136532