https://bugs.llvm.org/show_bug.cgi?id=43007
Bug ID: 43007
Summary: Missed Tail Call Optimization when specifying function
return value with __builtin_unreachable or
__builtin_assume
Product: libraries
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: mik...@digitalcarbide.com
CC: llvm-bugs@lists.llvm.org
When explicitly specifying that a function can only return a specific value
using either __builtin_unreachable or __builtin_assume, the optimizer fails to
perform tail call optimization.
This is observed for all architectures.
Example:
extern int function_returns_only_1_or_doesnt_return(int, int);
int foo1(int a, int b) {
const int result = function_returns_only_1_or_doesnt_return(a, b);
if (result == 1) {
return result;
}
else {
__builtin_unreachable();
}
}
int foo2(int a, int b) {
const int result = function_returns_only_1_or_doesnt_return(a, b);
__builtin_assume(result == 1);
return result;
}
int foo3(int a, int b) {
return function_returns_only_1_or_doesnt_return(a, b);
}
For the flags '-O3' for an x86-64 target, this emits the following assembly:
foo1(int, int): # @foo1(int, int)
push rax
call function_returns_only_1_or_doesnt_return(int, int)
mov eax, 1
pop rcx
ret
foo2(int, int): # @foo2(int, int)
push rax
call function_returns_only_1_or_doesnt_return(int, int)
mov eax, 1
pop rcx
ret
foo3(int, int): # @foo3(int, int)
jmp function_returns_only_1_or_doesnt_return(int, int) # TAILCALL
It would be expected that all three would perform tail call optimization.
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