GNU Arm Embedded Toolchain

Optimization lost for tail-call to a noreturn function

Bug #1646605 reported by Gary Fuehrer
12
This bug affects 2 people
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GNU Arm Embedded Toolchain
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Bug Description

Correctly this:
void __attribute__((noinline)) bar() { __asm ("0: b.n 0b"); }
void Reset_Handler() { <any-not-need-LR-code>; bar(); }
 gives:
<Reset_Handler>:
  <any-not-need-LR-code>
  b.w <bar>

But either:
void __attribute__((noinline)) bar() { G: goto G; }
 or this:
void __attribute__((noinline, noreturn)) bar() { __asm ("0: b.n 0b"); }
 gives:
<Reset_Handler>:
  push {r3, lr}
  <any-not-need-LR-code>
  bl <bar>

Compiler versions 4_7-2013q3-20130916-win32 thru 5_4-2016q3-20160926-win32 (current)

Revision history for this message
Gary Fuehrer (gfuehrer) wrote :

And, a "b.n" should be emitted, if possible, not a "b.w".

Revision history for this message
Gary Fuehrer (gfuehrer) wrote :

David Brown (https://launchpad.net/~davidbrown) said on 2016-12-02:

Also if you write:

extern void foo(void);
void bar(void)
{
  foo();
  __builtin_unreachable();
}

Then the __builtin_unreachable() causes the "push {r4, lr}" instruction.

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