https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96305
Bug ID: 96305
Summary: Unnecessary signed x unsigned multiplication with
squares or signed variables
Product: gcc
Version: 7.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: petr at nejedli dot cz
Target Milestone: ---
In presence of a signed variable multiplied by itself, the compiler seems to
recognize that the result will necessarily be positive, then considers the
result as unsigned going forward, causing unnecessarily complicated code down
the line.
I have initially reproduced the issue on 7.2.1 for arm, but I have verified the
same issue happens in the latest supported by the gotbolt compiler.
---
[nenik@Pix2 ~]$ arm-none-eabi-gcc --version
arm-none-eabi-gcc (GNU Tools for Arm Embedded Processors 7-2017-q4-major) 7.2.1
20170904 (release) [ARM/embedded-7-branch revision 255204]
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
[nenik@Pix2 ~]$ cat mull-issue.c
inline int hmull(int a, int b) {
return ((long long)a * b) >> 32;
}
int compute(int a, int b) {
int t = hmull(a,a);
return hmull(t, b);
}
[nenik@Pix2 ~]$ arm-none-eabi-gcc -Os -S -mcpu=cortex-m3 mull-issue.c
[nenik@Pix2 ~]$ cat mull-issue.s
.cpu cortex-m3
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 4
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.file "mull-issue.c"
.text
.align 1
.global compute
.syntax unified
.thumb
.thumb_func
.fpu softvfp
.type compute, %function
compute:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
smull r2, r3, r0, r0
push {r4, r6, r7, lr}
asrs r7, r1, #31
mul r0, r3, r7
asrs r4, r3, #31
mla r0, r1, r4, r0
umull r2, r3, r3, r1
add r0, r0, r3
pop {r4, r6, r7, pc}
.size compute, .-compute
.ident "GCC: (GNU Tools for Arm Embedded Processors 7-2017-q4-major)
7.2.1 20170904 (release) [ARM/embedded-7-branch revision 255204]"
---
https://godbolt.org/z/v186Yz
Expected code should be pretty much:
smull r2, r3, r0, r0
smull r2, r0, r3, r1
bx lr
under the simple reasoning, that r3, after the first smull, would be, at most,
0x40000000 for any argument and thus while certainly positive, never having the
highest bit set. r4 after second asrs will always be zero and so would be the
multiplicative part of the following mla, removing the need to go with umull
and fixing the result.
I have got clang to generate optimal code in a more complicated piece of SW.
I can also get gcc to generate two smulls (and smaller code overall) if I add
an unknown extra argument (or even a small constant) to the "t" variable before
the second hmull call, but if I try with a constant of zero and the compiler
manages to learn that, it gets back to suboptimal code.
