This patch tweaks RTL expansion of multi-word shifts and rotates to use
PLUS rather than IOR for disjunctive operations. During expansion of
these operations, the middle-end creates RTL like (X<<C1) | (Y>>C2)
where the constants C1 and C2 guarantee that bits don't overlap.
Hence the IOR can be performed by any any_or_plus operation, such as
IOR, XOR or PLUS; for word-size operations where carry chains aren't
an issue these should all be equally fast (single-cycle) instructions.
The benefit of this change is that targets with shift-and-add insns,
like x86's lea, can benefit from the LSHIFT-ADD form.
An example of a backend that benefits is ARC, which is demonstrated
by these two simple functions:
unsigned long long foo(unsigned long long x) { return x<<2; }
which with -O2 is currently compiled to:
foo: lsr r2,r0,30
asl_s r1,r1,2
asl_s r0,r0,2
j_s.d [blink]
or_s r1,r1,r2
with this patch becomes:
foo: lsr r2,r0,30
add2 r1,r2,r1
j_s.d [blink]
asl_s r0,r0,2
unsigned long long bar(unsigned long long x) { return (x<<2)|(x>>62); }
which with -O2 is currently compiled to 6 insns + return:
bar: lsr r12,r0,30
asl_s r3,r1,2
asl_s r0,r0,2
lsr_s r1,r1,30
or_s r0,r0,r1
j_s.d [blink]
or r1,r12,r3
with this patch becomes 4 insns + return:
bar: lsr r3,r1,30
lsr r2,r0,30
add2 r1,r2,r1
j_s.d [blink]
add2 r0,r3,r0
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures. Ok for mainline?
2024-01-18 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* expmed.cc (expand_shift_1): Use add_optab instead of ior_optab
to generate PLUS instead or IOR when unioning disjoint bitfields.
* optabs.cc (expand_subword_shift): Likewise.
(expand_binop): Likewise for double-word rotate.
Thanks in advance,
Roger
--
diff --git a/gcc/expmed.cc b/gcc/expmed.cc
index 5916d6ed1bc..d1900f97f0c 100644
--- a/gcc/expmed.cc
+++ b/gcc/expmed.cc
@@ -2610,10 +2610,11 @@ expand_shift_1 (enum tree_code code, machine_mode mode,
rtx shifted,
else if (methods == OPTAB_LIB_WIDEN)
{
/* If we have been unable to open-code this by a rotation,
- do it as the IOR of two shifts. I.e., to rotate A
- by N bits, compute
+ do it as the IOR or PLUS of two shifts. I.e., to rotate
+ A by N bits, compute
(A << N) | ((unsigned) A >> ((-N) & (C - 1)))
- where C is the bitsize of A.
+ where C is the bitsize of A. If N cannot be zero,
+ use PLUS instead of IOR.
It is theoretically possible that the target machine might
not be able to perform either shift and hence we would
@@ -2650,8 +2651,9 @@ expand_shift_1 (enum tree_code code, machine_mode mode,
rtx shifted,
temp1 = expand_shift_1 (left ? RSHIFT_EXPR : LSHIFT_EXPR,
mode, shifted, other_amount,
subtarget, 1);
- return expand_binop (mode, ior_optab, temp, temp1, target,
- unsignedp, methods);
+ return expand_binop (mode,
+ CONST_INT_P (op1) ? add_optab : ior_optab,
+ temp, temp1, target, unsignedp, methods);
}
temp = expand_binop (mode,
diff --git a/gcc/optabs.cc b/gcc/optabs.cc
index ce91f94ed43..dcd3e406719 100644
--- a/gcc/optabs.cc
+++ b/gcc/optabs.cc
@@ -566,8 +566,8 @@ expand_subword_shift (scalar_int_mode op1_mode, optab
binoptab,
if (tmp == 0)
return false;
- /* Now OR in the bits carried over from OUTOF_INPUT. */
- if (!force_expand_binop (word_mode, ior_optab, tmp, carries,
+ /* Now OR/PLUS in the bits carried over from OUTOF_INPUT. */
+ if (!force_expand_binop (word_mode, add_optab, tmp, carries,
into_target, unsignedp, methods))
return false;
}
@@ -1937,7 +1937,7 @@ expand_binop (machine_mode mode, optab binoptab, rtx op0,
rtx op1,
NULL_RTX, unsignedp, next_methods);
if (into_temp1 != 0 && into_temp2 != 0)
- inter = expand_binop (word_mode, ior_optab, into_temp1, into_temp2,
+ inter = expand_binop (word_mode, add_optab, into_temp1, into_temp2,
into_target, unsignedp, next_methods);
else
inter = 0;
@@ -1953,7 +1953,7 @@ expand_binop (machine_mode mode, optab binoptab, rtx op0,
rtx op1,
NULL_RTX, unsignedp, next_methods);
if (inter != 0 && outof_temp1 != 0 && outof_temp2 != 0)
- inter = expand_binop (word_mode, ior_optab,
+ inter = expand_binop (word_mode, add_optab,
outof_temp1, outof_temp2,
outof_target, unsignedp, next_methods);
