[LLVMdev] ARM Jump table pcrelative relaxation in clang (original) (raw)

Eric Bentura ebentura at gmail.com
Tue Jul 7 06:06:08 PDT 2015

I have created a small ll file to reproduce the problem. I used the intrinsic function llvm.arm.space to introduce space between the beginning of the code and the jump table. If the first argument of llvm.arm.space is higher than INT_MAX ( 2147483647), then the bug is hit. Lower or equal to that value, it passes. It looks like a precision issue. Does this sound familiar to someone?

; ModuleID = 'test.c' target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64" target triple = "armv5e-none-linux-androideabi"

declare i32 @llvm.arm.space(i32, i32)

; Function Attrs: nounwind define i32 @main() #0 { entry: %retval = alloca i32, align 4 %a = alloca i32, align 4 store i32 0, i32* %retval store i32 0, i32* %a, align 4 %0 = load i32* %a, align 4 call i32 @llvm.arm.space(i32* 2147483647*, i32 undef) switch i32 %0, label %sw.default [ i32 0, label %sw.bb i32 1, label %sw.bb1 i32 2, label %sw.bb2 i32 3, label %sw.bb3 ]

sw.bb: ; preds = %entry store i32 1, i32* %retval br label %return

sw.bb1: ; preds = %entry store i32 2, i32* %retval br label %return

sw.bb2: ; preds = %entry store i32 3, i32* %retval br label %return

sw.bb3: ; preds = %entry store i32 4, i32* %retval br label %return

sw.default: ; preds = %entry br label %sw.epilog

sw.epilog: ; preds = %sw.default store i32 0, i32* %retval br label %return

return: ; preds = %sw.epilog, %sw.bb3, %sw.bb2, %sw.bb1, %sw.bb %2 = load i32* %retval ret i32 %2 }

; Function Attrs: nounwind declare i32 @rand() #0

attributes #0 = { nounwind "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="true" } attributes #1 = { nounwind }

!llvm.module.flags = !{!0, !1, !2} !llvm.ident = !{!3}

!0 = !{i32 1, !"wchar_size", i32 4} !1 = !{i32 1, !"min_enum_size", i32 4} !2 = !{i32 1, !"PIC Level", i32 1} !3 = !{!"clang version 3.7.0 (trunk 229364)"}

2015-07-06 21:13 GMT+03:00 Eric Bentura <ebentura at gmail.com>:

It is certainly helping - Thanks Renato.

2015-07-06 18:39 GMT+03:00 Renato Golin <renato.golin at linaro.org>:

On 6 July 2015 at 16:32, Eric Bentura <ebentura at gmail.com> wrote: > I tried to build the object file using clang 3.7 and it fails with the same > error. > Where should I look at in the ARM backend to understand what happens? > Where the jump table instruction is generated and supposed to be relaxed?

Have a look at lib/Target/ARM/ARMConstantIslandPass.cpp, especially where Tim's patch touches: http://llvm.org/viewvc/llvm-project?view=revision&revision=238680 Instruction relaxation rules should be in the TableGen files, I think, but that means it could be in a number of places. Step through lib/Target/ARM/ARMAsmPrinter.cpp, at ARMAsmPrinter::EmitJumpTableInsts and see what the operand is. You'd expect that it would be already relaxed by that point. If it is, the bug is in the printer. If not, it could be in the instruction selection process, either ARMISelLowering or during validation, at ARMISelDAGToDAG. Hope that helps. cheers, --renato -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150707/a158a1e2/attachment.html>

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