https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93487
Bug ID: 93487
Summary: Missed tail-call optimizations
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: pskocik at gmail dot com
Target Milestone: ---
Given, for example:
#include <stdint.h>
typedef union lp_tp {
long l;
void *p;
} lp_tp ;
typedef union ilp_tp{
int i;
long l;
void *p;
lp_tp lp;
} ilp_tp;
long lcallee(void);
int icallee(void);
void *pcallee(void);
lp_tp lpcallee(void);
//tail calls on clang but not on gcc
int l2i(void){ return lcallee(); }
ilp_tp l_caller(void) { long rc = lcallee(); return (ilp_tp){.l=rc}; }
ilp_tp p_caller(void) { void *rc= pcallee(); return (ilp_tp){.p=rc}; }
ilp_tp lp_caller(void) { lp_tp rc = lpcallee(); return (ilp_tp){.lp =
rc}; }
ilp_tp lp_caller2(void) { lp_tp rc = lpcallee(); return (ilp_tp){.p =
rc.p}; }
struct foo* p2p_caller(void) { return pcallee(); } //optimized on both
uintptr_t p2up_caller(void) { return (uintptr_t)pcallee(); }
//optimized on both
//not optimized by either
ilp_tp i_caller(void) { int rc = icallee(); ilp_tp r; r.i=rc; return r;
}
clang (x86_64) is able to turn all of these calls except the last one into tail
calls but gcc tailcall-optimizes only the pointer-to-pointer conversions.
https://gcc.godbolt.org/z/Lw9-D2
