Jeff Law <l...@redhat.com> writes: > On 11/13/20 1:21 AM, Richard Sandiford via Gcc-patches wrote: >> This patch adds a routine for finding a “simple” SET for a register >> definition. See the comment in the patch for details. >> >> gcc/ >> * rtl.h (simple_regno_set): Declare. >> * rtlanal.c (simple_regno_set): New function. > So I was a bit confused that this is supposed to reject read-write, but > what it's really rejecting is a narrow subset of read-write. In > particular it rejects things that are potentially RMW via subregs. It > doesn't prevent the destination from appearing as a source operand. You > might consider clarifying the comment.
Yeah, in hindsight it was a mistake to spell out the RMW point separately when it was really just an extra condition on the subreg. I'd tweaked this comment and the mux-utils.h one (in response to Martin's feedback) while doing the cross-target testing, but forgot to include the changes to the committed version. (The tested versions were otherwise identical, honest.) Tested on aarch64-linux-gnu and x86_64-linux-gnu. OK to install? Thanks, Richard gcc/ * mux-utils.h (pointer_mux::m_ptr): Tweak description of contents. * rtlanal.c (simple_regno_set): Tweak description to clarify the RMW condition. --- gcc/mux-utils.h | 8 ++++---- gcc/rtlanal.c | 8 +++++--- 2 files changed, 9 insertions(+), 7 deletions(-) diff --git a/gcc/mux-utils.h b/gcc/mux-utils.h index b026a9fa4c1..6ec0669256d 100644 --- a/gcc/mux-utils.h +++ b/gcc/mux-utils.h @@ -139,10 +139,10 @@ public: private: pointer_mux (char *ptr) : m_ptr (ptr) {} - // The pointer value for A pointers, or the pointer value + 1 for B pointers. - // Using a pointer rather than a uintptr_t tells the compiler that second () - // can never return null, and that second_or_null () is only null if - // is_first (). + // Points to the first byte of an object for A pointers or the second + // byte of an object for B pointers. Using a pointer rather than a + // uintptr_t tells the compiler that second () can never return null, + // and that second_or_null () is only null if is_first (). char *m_ptr; }; diff --git a/gcc/rtlanal.c b/gcc/rtlanal.c index 80e72d6049d..f0e66a7b26b 100644 --- a/gcc/rtlanal.c +++ b/gcc/rtlanal.c @@ -1458,11 +1458,13 @@ set_of (const_rtx pat, const_rtx insn) /* Check whether instruction pattern PAT contains a SET with the following properties: - - the SET is executed unconditionally; - - the destination of the SET is write-only rather than read-write; and + - the SET is executed unconditionally; and - either: - the destination of the SET is a REG that contains REGNO; or - - the destination of the SET is a SUBREG of such a REG. + - both: + - the destination of the SET is a SUBREG of such a REG; and + - writing to the subreg clobbers all of the SUBREG_REG + (in other words, read_modify_subreg_p is false). If PAT does have a SET like that, return the set, otherwise return null.