In this PR we have two return paths from a function "map". The common code sets <result> to the value returned by one path, while the other path does:
<retval> = map (&<retval>, ...); We treated this call as tail recursion, losing the copy semantics on the value returned by the recursive call. We'd correctly reject the same thing for variables: local = map (&local, ...); The problem is that RESULT_DECLs didn't get the same treatment. Tested on aarch64-linux-gnu and x86_64-linux-gnu. OK to install? Richard 2019-08-09 Richard Sandiford <richard.sandif...@arm.com> gcc/ PR middle-end/90313 * tree-tailcall.c (find_tail_calls): Reject calls that might read from an escaped RESULT_DECL. gcc/testsuite/ PR middle-end/90313 * g++.dg/torture/pr90313.cc: New test. Index: gcc/tree-tailcall.c =================================================================== --- gcc/tree-tailcall.c 2019-05-29 10:49:37.868706770 +0100 +++ gcc/tree-tailcall.c 2019-08-09 09:31:27.441318174 +0100 @@ -491,6 +491,35 @@ find_tail_calls (basic_block bb, struct && !stmt_can_throw_external (cfun, stmt)) return; + /* If the function returns a value, then at present, the tail call + must return the same type of value. There is conceptually a copy + between the object returned by the tail call candidate and the + object returned by CFUN itself. + + This means that if we have: + + lhs = f (&<retval>); // f reads from <retval> + // (lhs is usually also <retval>) + + there is a copy between the temporary object returned by f and lhs, + meaning that any use of <retval> in f occurs before the assignment + to lhs begins. Thus the <retval> that is live on entry to the call + to f is really an independent local variable V that happens to be + stored in the RESULT_DECL rather than a local VAR_DECL. + + Turning this into a tail call would remove the copy and make the + lifetimes of the return value and V overlap. The same applies to + tail recursion, since if f can read from <retval>, we have to assume + that CFUN might already have written to <retval> before the call. + + The problem doesn't apply when <retval> is passed by value, but that + isn't a case we handle anyway. */ + tree result_decl = DECL_RESULT (cfun->decl); + if (result_decl + && may_be_aliased (result_decl) + && ref_maybe_used_by_stmt_p (call, result_decl)) + return; + /* We found the call, check whether it is suitable. */ tail_recursion = false; func = gimple_call_fndecl (call); Index: gcc/testsuite/g++.dg/torture/pr90313.cc =================================================================== --- /dev/null 2019-07-30 08:53:31.317691683 +0100 +++ gcc/testsuite/g++.dg/torture/pr90313.cc 2019-08-09 09:31:27.437318206 +0100 @@ -0,0 +1,33 @@ +// { dg-do run } + +#include <stddef.h> + +namespace std { + template<typename T, size_t N> struct array { + T elems[N]; + const T &operator[](size_t i) const { return elems[i]; } + }; +} + +using Coordinates = std::array<double, 3>; + +Coordinates map(const Coordinates &c, size_t level) +{ + Coordinates result{ c[1], c[2], c[0] }; + + if (level != 0) + result = map (result, level - 1); + + return result; +} + +int main() +{ + Coordinates vecOfCoordinates = { 1.0, 2.0, 3.0 }; + + auto result = map(vecOfCoordinates, 1); + if (result[0] != 3 || result[1] != 1 || result[2] != 2) + __builtin_abort (); + + return 0; +}