[testsuite] Test case for PR rtl-optimization/60473

[Richard Biener]

On Sun, Aug 2, 2020 at 6:31 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
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> PR rtl-optimization/60473 was a code quality regression that has
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> been cured by improvements to register allocation.  For the function
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> in the test case, GCC 4.4, 4.5 and 4.6 generated very poor code
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> requiring two mov instructions:
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> foo:    rdtsc
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>         mov     rcx, rax
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>         mov     rax, rdx
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>         sal     rax, 32
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>         or      rax, rcx
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>         ret
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> GCC 4.7 and 4.8 (when the PR was filed) produced better but
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> still poor code with one mov instruction:
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> foo:    rtdsc
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>         sal     rdx, 32
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>         or      rdx, rax
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>         mov     rax, rdx
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>         ret
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> Since GCC 4.9 (including current mainline), it generates optimal
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> code with no mov instructions, matching what used to be generated
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> in GCC 4.1.
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> foo:    rdtsc
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>         sal     rdx, 32
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>         or      rax, rdx
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>         ret
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> This test case, which has been tested on x86_64-pc-linux-gnu,
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> simply checks that we don't regress again in future.
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> Ok for mainline?

I think you need to restrict this to lp64, at least it fails for me with -m32.
It also passes with -mx32 but not sure how the perfect target selector
would look like.

Otherwise looks OK.

Richard.

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> 2020-08-02  Roger Sayle  <roger@nextmovesoftware.com>
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> gcc/testsuite/ChangeLog
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> PR rtl-optimization/60473
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> * gcc.target/i386/pr60473.c: New test.
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> Thanks in advance,
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> Roger
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> --
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> Roger Sayle
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> NextMove Software
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> Cambridge, UK
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