https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119120
--- Comment #5 from GCC Commits ---
The master branch has been updated by Jakub Jelinek :
https://gcc.gnu.org/g:d0d7c6632c2913c0243f048a15ff64aec6b6232e
commit r15-8059-gd0d7c6632c2913c0243f048a15ff64aec6b6232e
Author: Jakub Jelinek
Date: Fri Mar 14 15:31:47 2025 +0100
c, c++: Set DECL_NOT_GIMPLE_REG_P on *PART_EXPR operand on lhs of
MODIFY_EXPR [PR119120]
The PR119190 patch I've posted regresses the PR119120 testcase (not adding
to testsuite, as it is fairly hard to scan for that problem).
The issue is that for the partial setting of _Complex floating vars
through __real__ on it first and __imag__ later (or vice versa) and since
we forced all complex vars into SSA form we often have undefined (D)
arguments of those COMPLEX_EXPRs. When we don't DCE them (for -O0 debug
info reasons), their expansion will copy both the real and imag parts
using the floating mode and on some targets like 387 that copying alone can
unfortunately trigger exceptions on sNaNs or other problematic bit patterns
and for uninitialized memory it can be triggered randomly based on whatever
is on the stack before.
The following patch sets DECL_NOT_GIMPLE_REG_P flag in the FEs during
genericization.
I think Fortran doesn't have a way to modify just real or just complex
part separately.
The patch is for code like
_ComplexT __t;
__real__ __t = __z.real();
__imag__ __t = __z.imag();
_M_value *= __t;
return *this;
at -O0 which used to appear widely even in libstdc++ before GCC 9
and happens in real-world code. At -O0 for debug info reasons (see
PR119190) we don't want to aggressively DCE statements and when we
since r0-100845 try to rewrite vars with COMPLEX_TYPE into SSA form
aggressively, the above results in copying of uninitialized data
when expanding COMPLEX_EXPRs added so that the vars can be in SSA form.
The patch detects during genericization the partial initialization and
doesn't rewrite such vars to SSA at -O0. This has to be done before
gimplification starts, otherwise e.g. the attached testcase ICEs.
2025-03-14 Jakub Jelinek
PR target/119120
* c-gimplify.cc (c_genericize_control_r): Set DECL_NOT_GIMPLE_REG_P
on {REAL,IMAG}PART_EXPR is_gimple_reg operand at -O0 if it is lhs
of a MODIFY_EXPR.
* cp-gimplify.cc (cp_genericize_r): Set DECL_NOT_GIMPLE_REG_P
on {REAL,IMAG}PART_EXPR is_gimple_reg operand at -O0 if it is lhs
of a MODIFY_EXPR.
* c-c++-common/pr119120.c: New test.
