[patch 79279] combine/simplify_set issue

[Aurelien Buhrig <aurelien dot buhrig dot gcc at gmail dot com>]

Hi,

This patch fixes a combiner bug in simplify_set which calls
CANNOT_CHANGE_MODE_CLASS with wrong mode params.
It occurs when trying to simplify paradoxical subregs of hw regs (whose
natural mode is lower than a word).

In fact, changing from (set x:m1 (subreg:m1 (op:m2))) to (set (subreg:m2
x)  op2) is valid if REG_CANNOT_CHANGE_MODE_P (x, m1, m2) is false
and REG_CANNOT_CHANGE_MODE_P (x, GET_MODE (src), GET_MODE (SUBREG_REG
(src))

   See:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79279

Validated on a private target,
bootstraped on x86_64-pc-linux-gnu, but I'm not sure which target is the
most relevant for this patch though ...

OK to commit?

Aurélien

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