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perf: fix LTO build with gcc (hybrid VM) #23602

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perf: fix LTO build with gcc (hybrid VM) #23602

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1cff7a4
perf: fix LTO build with gcc (hybrid VM)
henderkes Sep 5, 2026
f0eff01
add i386 registers
henderkes Sep 6, 2026
689a527
feedback: add extra host handling in global reg var checks
henderkes Sep 6, 2026
07d96e5
capitalise lto both times
henderkes Sep 6, 2026
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18 changes: 18 additions & 0 deletions Zend/Zend.m4
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Original file line number Diff line number Diff line change
Expand Up @@ -330,6 +330,24 @@ AS_VAR_IF([php_cv_have_global_register_vars], [yes],
])
AC_MSG_CHECKING([whether to enable global register variables support])
AC_MSG_RESULT([$ZEND_GCC_GLOBAL_REGS])

dnl GCC doesn't propagate -ffixed-* from LTO objects. Reserve the VM registers
dnl before LTO code gen to avoid "global register variable follows a function definition"
AS_VAR_IF([ZEND_GCC_GLOBAL_REGS], [yes], [
zend_lto=no
for zend_flag in $CC $CFLAGS $LDFLAGS; do
AS_CASE([$zend_flag], [-flto|-flto=*], [zend_lto=yes], [-fno-lto], [zend_lto=no])
done
AS_VAR_IF([zend_lto], [yes], [
AS_CASE([$host_cpu],
[x86_64*|amd64*], [AS_VAR_APPEND([LDFLAGS], [" -ffixed-r14 -ffixed-r15"])],
[x86*|amd*|i?86*|pentium], [AS_VAR_APPEND([LDFLAGS], [" -ffixed-esi -ffixed-edi"])],
[aarch64*|arm64*], [AS_VAR_APPEND([LDFLAGS], [" -ffixed-x27 -ffixed-x28"])],
[ppc64*|powerpc64*], [AS_VAR_APPEND([LDFLAGS], [" -ffixed-r14 -ffixed-r15"])],

@devnexen devnexen Sep 6, 2026

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I m certain powerpc is correct, I was digging a bit about riscv64 and it looks correct too according to this doc, those registers are equivalent to s2/s3 confirmed with the gcc header (see REGISTER_NAMES/ADDITIONAL_REGISTER_NAMES).

@henderkes henderkes Sep 6, 2026

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I certainly hope they're correct, because otherwise php would crash, I copied them from

php-src/Zend/zend_execute.c

Line 52 in 810a9e5

# if defined(__GNUC__) && ZEND_GCC_VERSION >= 4008 && defined(i386)
😄

[riscv64*], [AS_VAR_APPEND([LDFLAGS], [" -ffixed-x18 -ffixed-x19"])],
[AC_MSG_ERROR([Cannot reserve VM registers for LTO, disable LTO or use --disable-gcc-global-regs])])
])
])
])

dnl
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2 changes: 2 additions & 0 deletions build/ltmain.sh
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Expand Up @@ -1689,6 +1689,8 @@ EOF
# -p, -pg, --coverage, -fprofile-* pass through profiling flag for GCC
# -F/path gives path to uninstalled frameworks, gcc on darwin
# @file GCC response files
# -ffixed-*, -fno-lto GCC LTO register reservations and cancellation
-ffixed-*|-fno-lto| \
Comment on lines +1692 to +1693

@arnaud-lb arnaud-lb Sep 7, 2026

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We should avoid modifying ltmain.sh directly as it's a generated/3rd party file. The version on master is much more recent, so it's possible that these changes are not necessary on that branch.

@henderkes henderkes Sep 7, 2026

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The changes are required there too, -ffixed is missing. (-fno-lto isn't required, I've only added it because -flto is in the line above, so passing "-flto -fno-lto" would create unexpected outcomes)

@arnaud-lb arnaud-lb Sep 7, 2026

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@petk @NattyNarwhal do you have thoughts about the ltmain.sh changes?

@NattyNarwhal NattyNarwhal Sep 7, 2026

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Next time libtool gets updated, it'll get clobbered. If this is a bug in libtool, we could carry it locally (and ship patches that could be applied), but you should probably seek to upstream it there.

@henderkes henderkes Sep 7, 2026 •
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Hmm, I'm not sure how to submit a patch to libtool:
image

Should I file a bug report instead and hope someone tends to it?

-64|-mips[0-9]|-r[0-9][0-9]*|-xarch=*|-xtarget=*|+DA*|+DD*|-q*|-m*| \
-t[45]*|-txscale*|-p|-pg|--coverage|-fprofile-*|-F*|@*)

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