Sorry about the delayed response but looking at the above discussion, should I
conclude that this is a valid tree simplification?
I am pasting the diff of the assembly that AArch64 generates with the test case
that I added. I see fewer instructions generated with the patch.
--- pr80131-1.s 2017-08-01 10:02:43.243374174 +0100
+++ pr80131-1.s-patched 2017-08-01 10:00:54.776455630 +0100
@@ -24,10 +24,8 @@
str x0, [sp, 8]
ldr x0, [sp, 8]
mov w1, w0
- mov w0, 63
- sub w0, w0, w1
- mov x1, 1
- lsl x0, x1, x0
+ mov x0, -9223372036854775808
+ lsr x0, x0, x1
add sp, sp, 16
ret
.size f2, .-f2
@@ -39,10 +37,8 @@
str x0, [sp, 8]
ldr x0, [sp, 8]
mov w1, w0
- mov w0, 63
- sub w0, w0, w1
- mov x1, 1
- lsl x0, x1, x0
+ mov x0, -9223372036854775808
+ lsr x0, x0, x1
add sp, sp, 16
ret
.size f3, .-f3
@@ -52,11 +48,9 @@
f4:
sub sp, sp, #16
str w0, [sp, 12]
- mov w1, 31
ldr w0, [sp, 12]
- sub w0, w1, w0
- mov w1, 1
- lsl w0, w1, w0
+ mov w1, -2147483648
+ lsr w0, w1, w0
add sp, sp, 16
ret
.size f4, .-f4
Thanks
Sudi
From: Wilco Dijkstra
Sent: Thursday, April 13, 2017 1:01 PM
To: Richard Biener; Jakub Jelinek
Cc: Sudi Das; GCC Patches; nd; Richard Earnshaw; James Greenhalgh
Subject: Re: [PATCH][GCC] Simplification of 1U << (31 - x)
Richard Biener wrote:
> It is IMHO a valid GIMPLE optimization / canonicalization.
>
> movabsq $-9223372036854775808, %rax
>
> so this should then have been generated as 1<<63?
>
> At some point variable shifts were quite expensive as well..
Yes I don't see a major difference between movabsq and
movl $1, %eax
salq $63, %rax
on my Sandy Bridge, but if the above is faster then that is what the x64
backend should emit - it's 1 byte smaller as well, so probably better in all
cases.
Wilco
