On 7/17/2026 3:40 PM, Matt Turner wrote:
On Fri, Jul 17, 2026, 4:06 PM Vladimir Makarov <[email protected]>
wrote:
On 7/15/26 2:19 PM, Matt Turner wrote:
> ira_setup_eliminable_regset calls df_set_regs_ever_live for the
hard frame
> pointer when it decides frame_pointer_needed. LRA can reach that
decision
> later instead, in setup_can_eliminate, when it finds the frame
pointer to
> stack pointer elimination is not possible after all, but it does
not mark the
> register live there.
>
> A target whose prologue decides which registers to save from
> df_regs_ever_live_p then sets up the frame pointer without
saving the caller's
> value. On alpha this miscompiles pge while bootstrapping the
Modula-2 front
> end (PR117184): alpha_compute_frame_layout leaves $15 out of the
save mask, so
> alpha_expand_prologue emits the "mov $30,$15" that clobbers it
but no matching
> store, while alpha_expand_epilogue restores the register whenever
> frame_pointer_needed, from an fp_offset that stayed 0 -- the
return address
> slot. The caller gets its call-saved $15 back as a code
address, which shows
> up much later as a NULL dereference, and the Modula-2 runtime
turns the
> resulting SIGSEGV into an unhandled exception:
>
> terminate called after throwing an instance of 'unsigned int'
>
> The testcase needs the VLA to reach the caller by inlining: a
caller with its
> own VLA has cfun->calls_alloca set, so IRA already knows a frame
pointer is
> needed and marks $15 live itself.
>
> Do what IRA does, so the two paths agree.
>
> gcc/
> * lra-eliminations.cc (setup_can_eliminate): Mark the hard
frame
> pointer live when setting frame_pointer_needed.
>
> gcc/testsuite/
> * gcc.target/alpha/frame-pointer-save-1.c: New test.
The patch looks good for me. You can commit it. Thank you for fixing
this.
Thank you!
I don't have commit access, so cc'ing Jeff Law.
Pushed. I don't have enough state on the PR to know if it's fully
resolved or not, so I left it alone.
jeff
