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* [Bug target/108987] New: [13 Regression] RISC-V: shiftadd cost model bug needlessly preferring syth_multiply
@ 2023-03-01 19:34 vineetg at rivosinc dot com
2023-03-01 19:56 ` [Bug target/108987] " vineetg at rivosinc dot com
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: vineetg at rivosinc dot com @ 2023-03-01 19:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108987
Bug ID: 108987
Summary: [13 Regression] RISC-V: shiftadd cost model bug
needlessly preferring syth_multiply
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: vineetg at rivosinc dot com
Target Milestone: ---
gcc trunk is preferring synthetic multiply using shift+add even when they are
costlier than multiply.
unsigned long long f5(unsigned long long i)
{
return i * 0x0202020202020202ULL;
}
riscv64-unknown-linux-gnu-gcc -c -O2 -march=rv64gc_zba
f5:
slli a5,a0,8
add a0,a5,a0
slli a5,a0,16
add a0,a0,a5
slli a5,a0,32
add a0,a0,a5
slli a0,a0,1
ret
With gcc 12.2 this used to be
f5:
lui a5,%hi(.LC0)
ld a5,%lo(.LC0)(a5)
mul a0,a0,a5
ret
This is a regression introduced by commit f90cb39235c4 ("RISC-V: costs: support
shift-and-add in strength-reduction"). It introduced the cost for
shift[1-3]+add (to favor SH*ADD) but due to a coding bug ended up doing this
for all shift values, affecting synth multiply among others.
This showed up as dynamic icount regression in SPEC 531.deepsjeng.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug target/108987] [13 Regression] RISC-V: shiftadd cost model bug needlessly preferring syth_multiply
2023-03-01 19:34 [Bug target/108987] New: [13 Regression] RISC-V: shiftadd cost model bug needlessly preferring syth_multiply vineetg at rivosinc dot com
@ 2023-03-01 19:56 ` vineetg at rivosinc dot com
2023-03-02 10:11 ` rguenth at gcc dot gnu.org
2023-03-06 21:50 ` vineetg at rivosinc dot com
2 siblings, 0 replies; 4+ messages in thread
From: vineetg at rivosinc dot com @ 2023-03-01 19:56 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108987
--- Comment #1 from Vineet Gupta <vineetg at rivosinc dot com> ---
Fix posted here
https://gcc.gnu.org/pipermail/gcc-patches/2023-March/613106.html
Essentially:
case PLUS:
if (TARGET_ZBA
&& mode == word_mode
&& GET_CODE (XEXP (x, 0)) == MULT
&& REG_P (XEXP (XEXP (x, 0), 0))
&& CONST_INT_P (XEXP (XEXP (x, 0), 1))
- && IN_RANGE (pow2p_hwi (INTVAL (XEXP (XEXP (x, 0), 1))), 1, 3))
+ && pow2p_hwi (INTVAL (XEXP (XEXP (x, 0), 1)))
+ && IN_RANGE (exact_log2 (INTVAL (XEXP (XEXP (x, 0), 1))), 1, 3))
{
*total = COSTS_N_INSNS (1);
return true;
}
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug target/108987] [13 Regression] RISC-V: shiftadd cost model bug needlessly preferring syth_multiply
2023-03-01 19:34 [Bug target/108987] New: [13 Regression] RISC-V: shiftadd cost model bug needlessly preferring syth_multiply vineetg at rivosinc dot com
2023-03-01 19:56 ` [Bug target/108987] " vineetg at rivosinc dot com
@ 2023-03-02 10:11 ` rguenth at gcc dot gnu.org
2023-03-06 21:50 ` vineetg at rivosinc dot com
2 siblings, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-03-02 10:11 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108987
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Target| |riscv
Target Milestone|--- |13.0
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug target/108987] [13 Regression] RISC-V: shiftadd cost model bug needlessly preferring syth_multiply
2023-03-01 19:34 [Bug target/108987] New: [13 Regression] RISC-V: shiftadd cost model bug needlessly preferring syth_multiply vineetg at rivosinc dot com
2023-03-01 19:56 ` [Bug target/108987] " vineetg at rivosinc dot com
2023-03-02 10:11 ` rguenth at gcc dot gnu.org
@ 2023-03-06 21:50 ` vineetg at rivosinc dot com
2 siblings, 0 replies; 4+ messages in thread
From: vineetg at rivosinc dot com @ 2023-03-06 21:50 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108987
Vineet Gupta <vineetg at rivosinc dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |FIXED
Status|UNCONFIRMED |RESOLVED
--- Comment #2 from Vineet Gupta <vineetg at rivosinc dot com> ---
This is now merged in trunk
commit 7e52f4420ffb0946dfc97704d72fa8aa67251495
Author: Vineet Gupta <vineetg@rivosinc.com>
Date: Wed Mar 1 11:53:15 2023 -0800
RISC-V: costs: miscomputed shiftadd_cost triggering synth_mult [PR/108987]
This showed up as dynamic icount regression in SPEC 531.deepsjeng with upstream
gcc (vs. gcc 12.2). gcc was resorting to synthetic multiply using shift+add(s)
even when multiply had clear cost benefit.
....
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2023-03-06 21:50 UTC | newest]
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2023-03-01 19:34 [Bug target/108987] New: [13 Regression] RISC-V: shiftadd cost model bug needlessly preferring syth_multiply vineetg at rivosinc dot com
2023-03-01 19:56 ` [Bug target/108987] " vineetg at rivosinc dot com
2023-03-02 10:11 ` rguenth at gcc dot gnu.org
2023-03-06 21:50 ` vineetg at rivosinc dot com
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