When optimizing for size with -Oz, setting a register can be minimized by
pushing an immediate value to the stack and popping it to the destination.
Alas the one general register that shouldn't be updated via the stack is
the stack pointer itself, where "pop %esp" can't be represented in GCC's
RTL ("use of a register mentioned in pre_inc, pre_dec, post_inc or
post_dec is not permitted within the same instruction").  This patch
fixes PR target/106122 by explicitly checking for SP_REG in the
problematic peephole2.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures.  Ok for mainline?


2022-06-30  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        PR target/106122
        * config/i386/i386.md (peephole2): Avoid generating pop %esp
        when optimizing for size.

gcc/testsuite/ChangeLog
        PR target/106122
        * gcc.target/i386/pr106122.c: New test case.


Thanks in advance,
Roger
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