Fix non-determinism in global placement caused by different FFT output values #4518
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The `fft_test` checks that the FFT output is deterministic despite using floating point values. Signed-off-by: Tim 'mithro' Ansell <[email protected]>
GCC per default enables one optimization for x86_64 that can change the result of floating-point operations: -ffp-contract=fast. This allows the compiler to do floating-point expression contraction such as combining multiplication and addition instructions with an FMA instruction. However, the FMA instruction omits the rounding done in the multiplication instruction, making the calculations produce different results for some input values. Disabling floating-point contraction with -ffp-contract=off makes GCC generate code that produces a consistent result for x86_64. See https://kristerw.github.io/2021/11/09/fp-contract/ The `fft_test` checks that this is working correctly. Signed-off-by: Tim 'mithro' Ansell <[email protected]>
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maliberty
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QuantamHD
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This is needed for floating point stability. The global placement depends on the output of the FFT library. Without `-ffp-contract=off` the current FFT library produces different results dependent on if the compiler is targeting an x86 architecture which has the "FMA instruction". The FMA instruction omits the rounding done in the multiplication instruction, making the calculations produce different results for some input values See https://kristerw.github.io/2021/11/09/fp-contract/ See also OpenROAD change @ The-OpenROAD-Project/OpenROAD#4518 Signed-off-by: Tim 'mithro' Ansell <[email protected]>
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This is needed for floating point stability. The global placement depends on the output of the FFT library. Without `-ffp-contract=off` the current FFT library produces different results dependent on if the compiler is targeting an x86 architecture which has the "FMA instruction". The FMA instruction omits the rounding done in the multiplication instruction, making the calculations produce different results for some input values See https://kristerw.github.io/2021/11/09/fp-contract/ See also OpenROAD change @ The-OpenROAD-Project/OpenROAD#4518 Signed-off-by: Tim Ansell <[email protected]>
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The global placement depends on the output of the FFT library. Without
-ffp-contract=offthe current FFT library produces different results dependent on if the compiler is targeting an x86 architecture which has the "FMA instruction".The FMA instruction omits the rounding done in the multiplication instruction, making the calculations produce different results for some input values
See https://kristerw.github.io/2021/11/09/fp-contract/
This pull request;
-ffp-contract=offon for the whole of OpenROAD to make sure the floating point values are deterministic.