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83b5e8407b
Summary:
This implements two hooks that attempt to avoid control flow for RISC-V. RISC-V
will lower SELECTs into control flow, which is not a great idea.
The hook `hasMultipleConditionRegisters()` turns off the following
DAGCombiner folds:
select(C0|C1, x, y) <=> select(C0, x, select(C1, x, y))
select(C0&C1, x, y) <=> select(C0, select(C1, x, y), y)
The second hook `setJumpIsExpensive` controls a flag that has a similar purpose
and is used in CodeGenPrepare and the SelectionDAGBuilder.
Both of these have the effect of ensuring more logic is done before fewer jumps.
Note: with the `B` extension, we may be able to lower select into a conditional
move instruction, so at some point these hooks will need to be guarded based on
enabled extensions.
Reviewed By: luismarques
Differential Revision: https://reviews.llvm.org/D79268
87 lines
2.4 KiB
LLVM
87 lines
2.4 KiB
LLVM
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
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; RUN: llc -mtriple=riscv32 -verify-machineinstrs < %s \
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; RUN: | FileCheck -check-prefix=RV32I %s
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; RUN: llc -mtriple=riscv64 -verify-machineinstrs < %s \
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; RUN: | FileCheck -check-prefix=RV64I %s
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;; There are a few different ways to lower (select (or A, B), X, Y). This test
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;; ensures that we do so with as few branches as possible.
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define signext i32 @select_of_or(i1 zeroext %a, i1 zeroext %b, i32 signext %c, i32 signext %d) nounwind {
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; RV32I-LABEL: select_of_or:
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; RV32I: # %bb.0:
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; RV32I-NEXT: or a1, a0, a1
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; RV32I-NEXT: mv a0, a2
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; RV32I-NEXT: bnez a1, .LBB0_2
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; RV32I-NEXT: # %bb.1:
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; RV32I-NEXT: mv a0, a3
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; RV32I-NEXT: .LBB0_2:
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; RV32I-NEXT: ret
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;
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; RV64I-LABEL: select_of_or:
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; RV64I: # %bb.0:
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; RV64I-NEXT: or a1, a0, a1
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; RV64I-NEXT: mv a0, a2
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; RV64I-NEXT: bnez a1, .LBB0_2
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; RV64I-NEXT: # %bb.1:
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; RV64I-NEXT: mv a0, a3
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; RV64I-NEXT: .LBB0_2:
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; RV64I-NEXT: ret
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%1 = or i1 %a, %b
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%2 = select i1 %1, i32 %c, i32 %d
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ret i32 %2
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}
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declare signext i32 @either() nounwind
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declare signext i32 @neither() nounwind
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define signext i32 @if_of_or(i1 zeroext %a, i1 zeroext %b) nounwind {
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; RV32I-LABEL: if_of_or:
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; RV32I: # %bb.0:
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; RV32I-NEXT: addi sp, sp, -16
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; RV32I-NEXT: sw ra, 12(sp)
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; RV32I-NEXT: or a0, a0, a1
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; RV32I-NEXT: addi a1, zero, 1
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; RV32I-NEXT: bne a0, a1, .LBB1_2
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; RV32I-NEXT: # %bb.1: # %if.then
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; RV32I-NEXT: call either
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; RV32I-NEXT: j .LBB1_3
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; RV32I-NEXT: .LBB1_2: # %if.else
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; RV32I-NEXT: call neither
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; RV32I-NEXT: .LBB1_3: # %if.end
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; RV32I-NEXT: lw ra, 12(sp)
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; RV32I-NEXT: addi sp, sp, 16
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; RV32I-NEXT: ret
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;
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; RV64I-LABEL: if_of_or:
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; RV64I: # %bb.0:
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; RV64I-NEXT: addi sp, sp, -16
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; RV64I-NEXT: sd ra, 8(sp)
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; RV64I-NEXT: or a0, a0, a1
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; RV64I-NEXT: addi a1, zero, 1
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; RV64I-NEXT: bne a0, a1, .LBB1_2
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; RV64I-NEXT: # %bb.1: # %if.then
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; RV64I-NEXT: call either
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; RV64I-NEXT: j .LBB1_3
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; RV64I-NEXT: .LBB1_2: # %if.else
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; RV64I-NEXT: call neither
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; RV64I-NEXT: .LBB1_3: # %if.end
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; RV64I-NEXT: ld ra, 8(sp)
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; RV64I-NEXT: addi sp, sp, 16
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; RV64I-NEXT: ret
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%1 = or i1 %a, %b
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br i1 %1, label %if.then, label %if.else
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if.then:
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%2 = tail call i32 @either()
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br label %if.end
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if.else:
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%3 = tail call i32 @neither()
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br label %if.end
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if.end:
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%4 = phi i32 [%2, %if.then], [%3, %if.else]
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ret i32 %4
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}
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