Chris Chrulski via llvm-dev
2017-Mar-20 16:27 UTC
[llvm-dev] -ffast-math optimizations impacted by symbol renaming in header files
Hi, I came across an issue where some optimizations that would normally be applied to standard math function calls are not getting applied when the –ffast-math option is enabled on the Clang command line on a Linux x86_64 target. I tracked down the issue to occurring because the –ffast-math option is triggering Clang to preprocess the math.h header file with the __FINITE_MATH_ONLY__ macro set to 1. In this case, the Linux header files are redirecting several math functions using the __asm__ extension to rename the symbol. An example of what the header files contains is: extern double exp (double) __asm__ ("" "__exp_finite") __attribute__ ((__nothrow__ )); In effect, ‘exp’ gets converted to be ‘__exp_finite’ in the IR. Because some of the optimizations like constant folding or vectorization are looking for the original function name, those optimizations do not get triggered under –ffast-math due to these alternative function names. Other affected functions include: acos, asin, exp2, log, log2, log10, and a few others. Anybody know if this is intentional, or have input regarding whether these optimizations should be extended to also look for the renamed versions of these functions? Thanks, Chris A couple of example cases: // File: test_folding.c // Constant fold will occur with: // clang -O2 -S -emit-llvm test_folding.c // Constant fold will not occur with: // clang -O2 -S -emit-llvm -ffast-math test_folding.c #include <math.h> double test_fold_exp() { return exp(0.0); } double test_fold_acos() { return acos(1.0); } ---------------------------------------------- // File: test_vectorize.c // Vectorization will occur with: // clang -O2 -S -emit-llvm -fno-math-errno test_vectorize.c // Vectorization will not occur with: // clang -O2 -S -emit-llvm -fno-math-errno -ffast-math test_vectorize.c #include <math.h> void test_vectorize_exp(long n, float* restrict y, float* restrict x) { long i; for (i = 0; i < n; i++) { x[i] = expf(y[i]); } }
Hal Finkel via llvm-dev
2017-Mar-20 20:20 UTC
[llvm-dev] -ffast-math optimizations impacted by symbol renaming in header files
On 03/20/2017 11:27 AM, Chris Chrulski via llvm-dev wrote:> Hi, > > I came across an issue where some optimizations that would normally be applied to standard math function calls are not getting applied when the –ffast-math option is enabled on the Clang command line on a Linux x86_64 target. > > I tracked down the issue to occurring because the –ffast-math option is triggering Clang to preprocess the math.h header file with the __FINITE_MATH_ONLY__ macro set to 1. In this case, the Linux header files are redirecting several math functions using the __asm__ extension to rename the symbol. An example of what the header files contains is: > extern double exp (double) __asm__ ("" "__exp_finite") __attribute__ ((__nothrow__ )); > > In effect, ‘exp’ gets converted to be ‘__exp_finite’ in the IR. Because some of the optimizations like constant folding or vectorization are looking for the original function name, those optimizations do not get triggered under –ffast-math due to these alternative function names. > > Other affected functions include: acos, asin, exp2, log, log2, log10, and a few others. > > Anybody know if this is intentional, or have input regarding whether these optimizations should be extended to also look for the renamed versions of these functions?It is not intentional; the optimizations should be extended. I imagine that we would extend TargetLibraryInfo to have names for these functions, and then we could recognize them when available, and generate them when available and the inputs have the 'fast' tag. -Hal> > Thanks, > Chris > > > > A couple of example cases: > > // File: test_folding.c > > // Constant fold will occur with: > // clang -O2 -S -emit-llvm test_folding.c > > // Constant fold will not occur with: > // clang -O2 -S -emit-llvm -ffast-math test_folding.c > > #include <math.h> > > double test_fold_exp() { > return exp(0.0); > } > > double test_fold_acos() { > return acos(1.0); > } > > ---------------------------------------------- > > // File: test_vectorize.c > > // Vectorization will occur with: > // clang -O2 -S -emit-llvm -fno-math-errno test_vectorize.c > > // Vectorization will not occur with: > // clang -O2 -S -emit-llvm -fno-math-errno -ffast-math test_vectorize.c > > #include <math.h> > > void test_vectorize_exp(long n, float* restrict y, float* restrict x) { > long i; > for (i = 0; i < n; i++) { > x[i] = expf(y[i]); > } > } > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev-- Hal Finkel Lead, Compiler Technology and Programming Languages Leadership Computing Facility Argonne National Laboratory
Chris Chrulski via llvm-dev
2017-Mar-20 22:41 UTC
[llvm-dev] -ffast-math optimizations impacted by symbol renaming in header files
Thanks Hal, I'll take a look into extending the TargetLibraryInfo. Chris On Monday, March 20, 2017 1:20 PM, Hal Finkel <hfinkel at anl.gov> wrote: On 03/20/2017 11:27 AM, Chris Chrulski via llvm-dev wrote:> Hi, > > I came across an issue where some optimizations that would normally be applied to standard math function calls are not getting applied when the –ffast-math option is enabled on the Clang command line on a Linux x86_64 target. > > I tracked down the issue to occurring because the –ffast-math option is triggering Clang to preprocess the math.h header file with the __FINITE_MATH_ONLY__ macro set to 1. In this case, the Linux header files are redirecting several math functions using the __asm__ extension to rename the symbol. An example of what the header files contains is: > extern double exp (double) __asm__ ("" "__exp_finite") __attribute__ ((__nothrow__ )); > > In effect, ‘exp’ gets converted to be ‘__exp_finite’ in the IR. Because some of the optimizations like constant folding or vectorization are looking for the original function name, those optimizations do not get triggered under –ffast-math due to these alternative function names. > > Other affected functions include: acos, asin, exp2, log, log2, log10, and a few others. > > Anybody know if this is intentional, or have input regarding whether these optimizations should be extended to also look for the renamed versions of these functions?It is not intentional; the optimizations should be extended. I imagine that we would extend TargetLibraryInfo to have names for these functions, and then we could recognize them when available, and generate them when available and the inputs have the 'fast' tag. -Hal> > Thanks, > Chris > > > > A couple of example cases: > > // File: test_folding.c > > // Constant fold will occur with: > // clang -O2 -S -emit-llvm test_folding.c > > // Constant fold will not occur with: > // clang -O2 -S -emit-llvm -ffast-math test_folding.c > > #include <math.h> > > double test_fold_exp() { > return exp(0.0); > } > > double test_fold_acos() { > return acos(1.0); > } > > ---------------------------------------------- > > // File: test_vectorize.c > > // Vectorization will occur with: > // clang -O2 -S -emit-llvm -fno-math-errno test_vectorize.c > > // Vectorization will not occur with: > // clang -O2 -S -emit-llvm -fno-math-errno -ffast-math test_vectorize.c > > #include <math.h> > > void test_vectorize_exp(long n, float* restrict y, float* restrict x) { > long i; > for (i = 0; i < n; i++) { > x[i] = expf(y[i]); > } > } > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev-- Hal Finkel Lead, Compiler Technology and Programming Languages Leadership Computing Facility Argonne National Laboratory -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170320/03435350/attachment.html>
Maybe Matching Threads
- Incorrect code generation when using -fprofile-generate on code which contains exception handling (Windows target)
- Incorrect code generation when using -fprofile-generate on code which contains exception handling (Windows target)
- LibCallAliasAnalysis class instantiation parameter
- Looping through file names and loading...
- [LLVMdev] problem trying to write an LLVM register-allocation pass