c-family: Tweak ptr +- (expr +- cst) FE optimization [PR120837]HEADtrunkmaster

The following testcase is miscompiled with -fsanitize=undefined but we
introduce UB into the IL even without that flag.

The optimization ptr +- (expr +- cst) when expr/cst have undefined
overflow into (ptr +- cst) +- expr is sometimes simply not valid,
without careful analysis on what ptr points to we don't know if it
is valid to do (ptr +- cst) pointer arithmetics.
E.g. on the testcase, ptr points to start of an array (actually
conditionally one or another) and cst is -1, so ptr - 1 is invalid
pointer arithmetics, while ptr + (expr - 1) can be valid if expr
is at runtime always > 1 and smaller than size of the array ptr points
to + 1.

Unfortunately, removing this 1992-ish optimization altogether causes
FAIL: c-c++-common/restrict-2.c  -Wc++-compat   scan-tree-dump-times lim2 "Moving statement" 11
FAIL: gcc.dg/tree-ssa/copy-headers-5.c scan-tree-dump ch2 "is now do-while loop"
FAIL: gcc.dg/tree-ssa/copy-headers-5.c scan-tree-dump-times ch2 "  if " 3
FAIL: gcc.dg/vect/pr57558-2.c scan-tree-dump vect "vectorized 1 loops"
FAIL: gcc.dg/vect/pr57558-2.c -flto -ffat-lto-objects  scan-tree-dump vect "vectorized 1 loops"
regressions (restrict-2.c also for C++ in all std modes).  I've been thinking
about some match.pd optimization for signed integer addition/subtraction of
constant followed by widening integral conversion followed by multiplication
or left shift, but that wouldn't help 32-bit arches.

So, instead at least for now, the following patch keeps doing the
optimization, just doesn't perform it in pointer arithmetics.
pointer_int_sum itself actually adds the multiplication by size_exp,
so ptr + expr is turned into ptr p+ expr * size_exp,
so this patch will try to optimize
ptr + (expr +- cst)
into
ptr p+ ((sizetype)expr * size_exp +- (sizetype)cst * size_exp)
and
ptr - (expr +- cst)
into
ptr p+ -((sizetype)expr * size_exp +- (sizetype)cst * size_exp)

2025-07-04  Jakub Jelinek  <jakub@redhat.com>

	PR c/120837
	* c-common.cc (pointer_int_sum): Rewrite the intop PLUS_EXPR or
	MINUS_EXPR optimization into extension of both intop operands,
	their separate multiplication and then addition/subtraction followed
	by rest of pointer_int_sum handling after the multiplication.

	* gcc.dg/ubsan/pr120837.c: New test.

0 files changed, 0 insertions, 0 deletions