Hi Jakub, On 12/12/22 14:56, Jakub Jelinek wrote:
On Mon, Dec 12, 2022 at 02:44:04PM +0100, Alejandro Colomar via Gcc wrote:I don't see any problem with the code snippets you provided.Well, then the optimization may be the other way around (although I question why it is implemented that way, and not the other way around, but I'm not a hardware or libc guy, so there may be reasons). If calling memcpy(3) is better, then the code calling mempcpy(3) could be expanded inline to call it (but I doubt it). If calling mempcpy(3) is better, then the hand-made pattern resembling mempcpy(3) should probably be merged as a call to mempcpy(3). But acting different on equivalent calls to both of them seems inconsistent to me, unless you trust the programmer to know better how to optimize, that is...I think that is the case, plus the question if one can use a non-standard function to implement a standard function (and if it would be triggered by seeing an expected prototype for the non-standard function).
I guess implementing a standard function by calling a non-standard one is fine. The implementation is free to do what it pleases, as long as it provides the expected interface.
Otherwise, whether mempcpy in libc is implemented as memcpy + tweak return value or has its own implementation is something that is heavily dependent on the target and changes over time, so hardcoding that in gcc is problematic.
Might be, although I'm guessing that if GCC collapses mempcpy(3)-like hand-made patterns to mempcpy(3), the worst that can happen is that glibc undoes that; not a horrible crime. In the best case, it saves a function call, or a few assignments.
For -Os mempcpy call might be very well smaller even if the library side is then slower.
Heh, you might be surprised with the following. Remember that the file ending in 1 is a hand-made pattern around memcpy(3), while the file ending in 3 calls mempcpy(3) directly; yet GCC emits more code for mempcpy(3). I don't see any reason for this.
Cheers,
Alex
---
$ diff -u usts2stp[13].s
--- usts2stp1.s 2022-12-12 15:00:34.775119720 +0100
+++ usts2stp3.s 2022-12-12 15:00:34.807119072 +0100
@@ -1,12 +1,13 @@
- .file "usts2stp1.c"
+ .file "usts2stp3.c"
.text
.globl usts2stp
.type usts2stp, @function
usts2stp:
.LFB0:
.cfi_startproc
- movq (%rsi), %rcx
+ movq %rsi, %rax
movq 8(%rsi), %rsi
+ movq (%rax), %rcx
rep movsb
movb $0, (%rdi)
movq %rdi, %rax
--
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